Solving Exponential Equations with Logarithms
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Algebra 2 › Solving Exponential Equations with Logarithms
Solve and evaluate: $2\cdot e^{0.5t}=20$. Write $t$ in exact logarithmic form and then approximate using technology.
$t=\ln(5)\approx 1.609$
$t=2\ln(10)\approx 4.605$
$t=\ln(10)\approx 2.303$
$t=2\ln(20)\approx 5.991$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. The solving strategy for ab^(ct) = d: (1) Isolate the exponential by dividing both sides by a: b^(ct) = d/a, (2) Take log base b of both sides: log_b(b^(ct)) = log_b(d/a), (3) Use inverse property on left: ct = log_b(d/a), (4) Solve for variable: t = log_b(d/a)/c. This systematic approach works for any exponential equation in this form! To solve $2·e^{0.5t}$=20, divide by 2: $e^{0.5t}$=10, take natural log: $ln(e^{0.5t}$)=ln(10), simplify to 0.5t=ln(10), and multiply by 2: t=2 ln(10). Choice A correctly isolates, takes the natural logarithm, applies the inverse property since $ln(e^y$)=y, and approximates to 4.605. Choice B forgets to multiply by 2, resulting in t=ln(10) which is half the correct value—be sure to solve for t by dividing by the coefficient 0.5, which is the same as multiplying by 2! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
Use a calculator to solve the exponential equation $4\cdot 2^{t/5}=30$. Give the exact solution as a logarithm and an approximate decimal value.
$t=5\log_2\left(\dfrac{15}{2}\right)\approx 14.535$
$t=\dfrac{\log_2\left(\dfrac{15}{2}\right)}{5}\approx 0.581$
$t=5\log\left(\dfrac{15}{2}\right)\approx 4.375$
$t=5\log_2\left(\dfrac{30}{4}\right)\approx 5.814$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. When solving exponential equations like $2^t$ = 10 where the exponent contains the variable, logarithms are the tool that unlocks the solution: taking log base 2 of both sides gives $log₂(2^t$) = log₂(10), and using the inverse property $log₂(2^t$) = t, we get t = log₂(10). This is the exact solution! To get a decimal approximation, use your calculator with change of base: log₂(10) = ln(10)/ln(2) ≈ 3.322. To solve $4·2^{t/5}$=30, divide by 4: $2^{t/5}$=30/4=15/2, take log base 2: $log₂(2^{t/5}$)=log₂(15/2), simplify to t/5=log₂(15/2), and multiply by 5: t=5 log₂(15/2). Choice A correctly isolates, takes log base 2, applies the inverse property, and approximates to 14.535 using change of base on a calculator. Choice B divides by 5 instead of multiplying, resulting in t=log₂(15/2)/5 which is one-fifth the size—remember that to undo division in the exponent, you multiply when solving for t! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
Solve for $x$ and evaluate using technology: $8\cdot 2^{x/2}=70$. Express the exact solution as a logarithm.
$x=\dfrac{\log_2\left(\dfrac{35}{4}\right)}{2}\approx 1.565$
$x=2\log_2\left(\dfrac{35}{4}\right)\approx 6.258$
$x=2\log_2\left(\dfrac{70}{8}\right)\approx 3.129$
$x=2\log\left(\dfrac{35}{4}\right)\approx 1.792$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. When solving exponential equations like $2^t$ = 10 where the exponent contains the variable, logarithms are the tool that unlocks the solution: taking log base 2 of both sides gives $log₂(2^t$) = log₂(10), and using the inverse property $log₂(2^t$) = t, we get t = log₂(10). This is the exact solution! To get a decimal approximation, use your calculator with change of base: log₂(10) = ln(10)/ln(2) ≈ 3.322. To solve $8·2^{x/2}$=70, divide by 8: $2^{x/2}$=70/8=35/4, take log base 2: $log₂(2^{x/2}$)=log₂(35/4), simplify to x/2=log₂(35/4), and multiply by 2: x=2 log₂(35/4). Choice A correctly isolates the exponential, takes log base 2, applies the inverse property, and approximates to 6.258 using change of base. Choice B divides by 2 instead of multiplying, giving x=log₂(35/4)/2 which is half the correct value—remember to multiply by 2 to undo the division in the exponent! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
Solve the exponential equation $3\cdot 10^{2x}=75$ for $x$. Give the exact solution (as a logarithm) and an approximate value using technology.
$x=\dfrac{\log(75)}{2}\approx 0.938$
$x=\log(25)\approx 1.398$
$x=\dfrac{\ln(25)}{2}\approx 1.609$
$x=\dfrac{\log(25)}{2}\approx 0.699$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. The solving strategy for ab^(ct) = d: (1) Isolate the exponential by dividing both sides by a: b^(ct) = d/a, (2) Take log base b of both sides: log_b(b^(ct)) = log_b(d/a), (3) Use inverse property on left: ct = log_b(d/a), (4) Solve for variable: t = log_b(d/a)/c. This systematic approach works for any exponential equation in this form! To solve $3·10^{2x}$=75, first divide both sides by 3 to isolate the exponential: $10^{2x}$=25, then take the common logarithm (base 10) of both sides: $log(10^{2x}$)=log(25), which simplifies to 2x=log(25) using the inverse property, and finally divide by 2: x=log(25)/2. Choice A correctly isolates and takes the logarithm to get the exact solution as log(25)/2 and approximates it to 0.699 using technology. Choice C forgets to divide by 2 after taking the log, resulting in x=log(25) which is twice as large as it should be—remember to solve fully for the variable after applying the logarithm! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
What is $t$ in the equation $7\cdot 10^{t/3}=50$? Express $t$ as a logarithm and evaluate using technology.
$t=3\log\left(\dfrac{7}{50}\right)\approx -2.571$
$t=3\log\left(\dfrac{50}{7}\right)\approx 2.571$
$t=\log\left(\dfrac{50}{7}\right)\approx 0.857$
$t=\log\left(\dfrac{50}{7}\right)^3\approx 0.629$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. The solving strategy for ab^(ct) = d: (1) Isolate the exponential by dividing both sides by a: b^(ct) = d/a, (2) Take log base b of both sides: log_b(b^(ct)) = log_b(d/a), (3) Use inverse property on left: ct = log_b(d/a), (4) Solve for variable: t = log_b(d/a)/c. This systematic approach works for any exponential equation in this form! To solve $7·10^{t/3}$=50, divide by 7: $10^{t/3}$=50/7, take common log: $log(10^{t/3}$)=log(50/7), simplify to t/3=log(50/7), and multiply by 3: t=3 log(50/7). Choice A correctly isolates the exponential, takes the common logarithm, uses the inverse property, and provides the exact form with an approximation. Choice B skips multiplying by 3, giving t=log(50/7) which is one-third the correct value—don't forget to multiply by the reciprocal of the exponent's coefficient! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
Use logarithms to solve the equation $2.5\cdot e^{t/4}=40$ for $t$. Give the exact solution and an approximate value using a calculator.
$t=\dfrac{\ln(16)}{4}\approx 0.693$
$t=4\ln(42.5)\approx 15.003$
$t=\ln(16)\approx 2.773$
$t=4\ln(16)\approx 11.090$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. The solving strategy for ab^(ct) = d: (1) Isolate the exponential by dividing both sides by a: b^(ct) = d/a, (2) Take log base b of both sides: log_b(b^(ct)) = log_b(d/a), (3) Use inverse property on left: ct = log_b(d/a), (4) Solve for variable: t = log_b(d/a)/c. This systematic approach works for any exponential equation in this form! To solve $2.5·e^{t/4}$=40, divide by 2.5: $e^{t/4}$=16, take natural log: $ln(e^{t/4}$)=ln(16), simplify to t/4=ln(16), and multiply by 4: t=4 ln(16). Choice A correctly isolates, takes the natural logarithm, uses the inverse property, and approximates to 11.090 using technology. Choice B divides by 4 instead of multiplying, resulting in t=ln(16)/4 which is one-fourth the value—always multiply by the reciprocal of the exponent's coefficient! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
Solve for $x$: $12 \cdot e^{2x} = 90$. Express the solution as a logarithm and approximate using technology.
$x = \dfrac{\log\left(\dfrac{15}{2}\right)}{2} \approx 0.438$
$x = \dfrac{\ln\left(\dfrac{15}{2}\right)}{2} \approx 1.007$
$x = \ln\left(\dfrac{15}{2}\right) \approx 2.015$
$x = \dfrac{\ln\left(\dfrac{2}{15}\right)}{2} \approx -1.007$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. The solving strategy for $ab^{ct} = d$: (1) Isolate the exponential by dividing both sides by $a$: $b^{ct} = \frac{d}{a}$, (2) Take log base $b$ of both sides: $\log_b(b^{ct}) = \log_b(\frac{d}{a})$, (3) Use inverse property on left: $ct = \log_b(\frac{d}{a})$, (4) Solve for variable: $t = \frac{\log_b(\frac{d}{a})}{c}$. This systematic approach works for any exponential equation in this form! To solve $12 \cdot e^{2x} = 90$, divide by 12: $e^{2x} = \frac{90}{12} = \frac{15}{2}$, take natural log: $\ln(e^{2x}) = \ln(\frac{15}{2})$, simplify to $2x = \ln(\frac{15}{2})$, and divide by 2: $x = \frac{\ln(\frac{15}{2})}{2}$. Choice A correctly isolates the exponential, takes the natural logarithm, applies the inverse property, and approximates to 1.007. Choice C omits dividing by 2, giving $x = \ln(\frac{15}{2})$ which is twice the correct value—always divide by the exponent's coefficient to isolate the variable! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need $\log_2(10)$? Use change of base: $\log_2(10) = \frac{\ln(10)}{\ln(2)}$ or $\frac{\log(10)}{\log(2)}$—both give the same answer ≈ 3.322. The formula is $\log_b(x) = \frac{\ln(x)}{\ln(b)}$ for any base $b$. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression $b^{ct}$ before taking logarithms. If you have $5 \cdot 2^t = 40$, first divide by 5 to get $2^t = 8$, THEN take log. Taking $\log_2$ of both sides of $5 \cdot 2^t = 40$ directly leads to $\log_2(5 \cdot 2^t)$, which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have $b^{\text{something}} = \text{number}$, with the exponential alone on one side!
Use logarithms to solve $5\cdot 2^{3t}=60$ for $t$. Express your answer as a logarithm and then approximate using a calculator.
$t=\dfrac{\log_2(12)}{5}\approx 0.717$
$t=\log_2(12)\approx 3.585$
$t=\dfrac{\log_2(60)}{3}\approx 1.969$
$t=\dfrac{\log_2(12)}{3}\approx 1.195$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. When solving exponential equations like $2^t$ = 10 where the exponent contains the variable, logarithms are the tool that unlocks the solution: taking log base 2 of both sides gives $log₂(2^t$) = log₂(10), and using the inverse property $log₂(2^t$) = t, we get t = log₂(10). This is the exact solution! To get a decimal approximation, use your calculator with change of base: log₂(10) = ln(10)/ln(2) ≈ 3.322. To solve $5·2^{3t}$=60, first divide both sides by 5 to isolate: $2^{3t}$=12, then take log base 2: $log₂(2^{3t}$)=log₂(12), which simplifies to 3t=log₂(12), and divide by 3: t=log₂(12)/3. Choice A correctly isolates the exponential, applies the logarithm base 2, uses the inverse property, and approximates to 1.195. Choice C omits dividing by 3, leaving t=log₂(12) which is three times larger than needed—always remember to divide by the coefficient in the exponent after taking the log! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
Find $t$ in $9\cdot 2^{4t}=100$. Express the solution as a logarithm (base 2) and approximate using a calculator.
$t=\dfrac{\log\left(\dfrac{100}{9}\right)}{4}\approx 0.261$
$t=\dfrac{\log_2\left(\dfrac{100}{9}\right)}{4}\approx 0.869$
$t=\dfrac{\log_2\left(\dfrac{9}{100}\right)}{4}\approx -0.869$
$t=\log_2\left(\dfrac{100}{9}\right)\approx 3.475$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. When solving exponential equations like $2^t$ = 10 where the exponent contains the variable, logarithms are the tool that unlocks the solution: taking log base 2 of both sides gives $log₂(2^t$) = log₂(10), and using the inverse property $log₂(2^t$) = t, we get t = log₂(10). This is the exact solution! To get a decimal approximation, use your calculator with change of base: log₂(10) = ln(10)/ln(2) ≈ 3.322. To solve $9·2^{4t}$=100, divide by 9: $2^{4t}$=100/9, take log base 2: $log₂(2^{4t}$)=log₂(100/9), simplify to 4t=log₂(100/9), and divide by 4: t=log₂(100/9)/4. Choice A correctly isolates, takes log base 2, uses the inverse property, and approximates to 0.869 using change of base. Choice D forgets to divide by 4, resulting in t=log₂(100/9) which is four times larger—make sure to divide by the coefficient in the exponent! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!
Solve $6\cdot 10^{0.2t}=48$ for $t$. Express your solution as a logarithm and then approximate using technology.
$t=5\log(48)\approx 8.406$
$t=\log(8)\approx 0.903$
$t=\dfrac{\log(8)}{5}\approx 0.181$
$t=5\log(8)\approx 4.515$
Explanation
This question tests your ability to solve exponential equations by taking logarithms of both sides and using the inverse relationship to isolate the variable. The solving strategy for ab^(ct) = d: (1) Isolate the exponential by dividing both sides by a: b^(ct) = d/a, (2) Take log base b of both sides: log_b(b^(ct)) = log_b(d/a), (3) Use inverse property on left: ct = log_b(d/a), (4) Solve for variable: t = log_b(d/a)/c. This systematic approach works for any exponential equation in this form! To solve $6·10^{0.2t}$=48, divide by 6: $10^{0.2t}$=8, take common log: $log(10^{0.2t}$)=log(8), simplify to 0.2t=log(8), and divide by 0.2 (or multiply by 5): t=5 log(8). Choice A correctly isolates the exponential, takes the common logarithm, applies the inverse property, and approximates to 4.515. Choice B divides by 5 instead of multiplying, giving t=log(8)/5 which is too small—remember that dividing by 0.2 is multiplying by 5 to solve for t! Calculator strategy for non-standard bases: your calculator has log (base 10) and ln (base e) buttons, but what if you need log₂(10)? Use change of base: log₂(10) = ln(10)/ln(2) or log(10)/log(2)—both give the same answer ≈ 3.322. The formula is log_b(x) = ln(x)/ln(b) for any base b. This lets you evaluate any logarithm using just the ln button! Alternatively, leave answers in exact log form if calculator evaluation isn't required. Isolation before logarithms: ALWAYS isolate the exponential expression b^(ct) before taking logarithms. If you have $5·2^t$ = 40, first divide by 5 to get $2^t$ = 8, THEN take log. Taking log₂ of both sides of $5·2^t$ = 40 directly leads to $log₂(5·2^t$), which is more complex (requires log properties). Simple isolation first makes the logarithm application clean: take log of both sides when you have b^(something) = number, with the exponential alone on one side!