Local Linearity and Linearization
Help Questions
AP Calculus BC › Local Linearity and Linearization
Let $r(1)=\tfrac12$ and $r'(1)=\tfrac12$. What is the linear approximation of $r$ at $x=1$?
$L(x)=1+\tfrac12(x-\tfrac12)$
$L(x)=\tfrac12-\tfrac12(x-1)$
$L(x)=\tfrac12+\tfrac12(x-1)$
$L(x)=\tfrac12+1(x-\tfrac12)$
$L(x)=\tfrac12+\tfrac12(x+1)$
Explanation
This linearization problem involves fractional values for both function value and derivative. Using L(x) = r(a) + r'(a)(x - a) with a = 1, r(1) = 1/2, and r'(1) = 1/2. The linear approximation becomes L(x) = 1/2 + (1/2)(x - 1). Both the function value and slope are 1/2, creating a tangent line that passes through (1, 1/2) with slope 1/2. This means the function increases by 1 unit for every 2 units of increase in x. Choice C uses -1/2 for the derivative, which would represent decreasing behavior. When both function value and derivative are fractions, maintain precision and verify signs throughout the calculation.
A function $a$ has $a(8)=-6$ and $a'(8)=0.2$. What is the linearization at $x=8$?
$L(x)=-6+0.2(x-8)$
$L(x)=8+0.2(x+6)$
$L(x)=-6-0.2(x-8)$
$L(x)=0.2-6(x-8)$
$L(x)=-6+8(x-0.2)$
Explanation
This linearization problem features a decimal derivative and negative function value. Using L(x) = a(b) + a'(b)(x - b) with b = 8, a(8) = -6, and a'(8) = 0.2. The linearization becomes L(x) = -6 + 0.2(x - 8), representing the tangent line at (8, -6). The positive derivative 0.2 indicates the function is increasing slowly near x = 8, rising by 0.2 units per unit increase in x. Even though the function value is negative, the positive derivative shows upward movement. Choice C uses -0.2 instead of +0.2, suggesting decreasing behavior rather than increasing. Check that derivative signs match the intended function behavior (increasing vs decreasing).
A concentration function $C$ satisfies $C(0)=8$ and $C'(0)=5$. What is the linear approximation near $t=0$?
$L(t)=0+5(t-8)$
$L(t)=8-5t$
$L(t)=5+8t$
$L(t)=8+5t$
$L(t)=8+0(t-5)$
Explanation
This problem asks for linearization at x = 0, which simplifies the algebraic form. Using L(t) = C(a) + C'(a)(t - a) with a = 0, C(0) = 8, and C'(0) = 5. The linearization becomes L(t) = 8 + 5(t - 0) = 8 + 5t. When linearizing at the origin, the (t - 0) term simplifies to just t, creating a clean linear function. The concentration starts at 8 when t = 0 and increases by 5 units per unit increase in time. Choice B incorrectly maintains the (t - 8) form, suggesting linearization at t = 8 rather than t = 0. Linearization at the origin produces the simple form f(0) + f'(0)x.
Given $b(\pi)=0$ and $b'(\pi)=-2$, which is the linear approximation near $x=\pi$?
$L(x)=\pi-2(x-0)$
$L(x)=0-\pi(x+2)$
$L(x)=2(x-\pi)$
$L(x)=-2(x-\pi)$
$L(x)=-2(x+\pi)$
Explanation
This problem involves linearization at x = $\pi$ with a zero function value. Using $L(x) = b(a) + b'(a)(x - a)$ with a = $\pi$, b($\pi$) = 0, and b'($\pi$) = -2. The linearization becomes $L(x) = 0 + (-2)(x - \pi) = -2(x - \pi)$. Since the function value at $\pi$ is zero, the tangent line passes through ($\pi$, 0) with slope -2. This represents a decreasing linear function that crosses the x-axis at x = $\pi$. Choice C uses +2 instead of -2, which would represent increasing behavior rather than decreasing. When the function value is zero, the linearization simplifies to just the derivative times $(x - a)$.
If $m(5)=-1$ and $m'(5)=3$, what is the tangent line equation at $x=5$?
$y=3-1(x-5)$
$y=-1+3(x-5)$
$y=-1-3(x-5)$
$y=5+3(x+1)$
$y=-1+5(x-3)$
Explanation
This linearization question follows the standard tangent line format. With m(5) = -1 and m'(5) = 3, we use the formula y = f(a) + f'(a)(x - a) at a = 5. Substituting gives y = -1 + 3(x - 5), which represents the tangent line at the point (5, -1). The linear approximation has slope 3, meaning the function increases by 3 units for each unit increase in x near x = 5. This tangent line provides the best linear estimate of m(x) in a neighborhood around x = 5. Choice C uses -3 instead of +3 for the slope, which would represent a decreasing function rather than increasing. Always verify: point coordinates, slope sign, and linearization point in the (x - a) term.
A temperature model $T$ satisfies $T(10)=68$ and $T'(10)=1.5$. What is the linear approximation near $t=10$?
$L(t)=1.5+68(t-10)$
$L(t)=10+1.5(t-68)$
$L(t)=68+1.5(t-10)$
$L(t)=68+10(t-1.5)$
$L(t)=68-1.5(t-10)$
Explanation
This question involves linearization of a temperature model at a specific time. The linear approximation formula L(t) = T(a) + T'(a)(t - a) applies here with a = 10. Given T(10) = 68 and T'(10) = 1.5, we substitute these values directly. The linearization becomes L(t) = 68 + 1.5(t - 10), representing how temperature changes near t = 10. This tangent line approximation uses the temperature value 68 and rate of change 1.5 at time t = 10. Choice B swaps the function value and point of linearization, which would be incorrect. Remember: linearization format is always function_value + derivative × (input - point).
If $g(1)=-2$ and $g'(1)=4$, which equation is the tangent line to $g$ at $x=1$?
$y=-2+4(x-1)$
$y=1+4(x+2)$
$y=4-2(x-1)$
$y=-2+1(x-4)$
$y=-2-4(x-1)$
Explanation
This problem asks for the tangent line equation, which is the same as linearization. The tangent line at x = a has equation y = f(a) + f'(a)(x - a). With g(1) = -2 and g'(1) = 4, we substitute a = 1 to get the equation. This yields y = -2 + 4(x - 1), which represents the linear approximation of g near x = 1. The tangent line passes through (1, -2) with slope 4, making it the best linear approximation in the neighborhood of x = 1. Choice B incorrectly uses (x + 2) instead of (x - 1), suggesting linearization at a different point. To check linearization: identify the point a, use f(a) as the constant term, and f'(a)(x - a) as the linear term.
A function $J$ satisfies $J(\tfrac12)=1$ and $J'(\tfrac12)=6$. What is the linear approximation near $x=\tfrac12$?
$L(x)=1-6\left(x-\tfrac12\right)$
$L(x)=6+1\left(x-\tfrac12\right)$
$L(x)=1+6\left(x-\tfrac12\right)$
$L(x)=\tfrac12+6(x-1)$
$L(x)=1+\tfrac12(x-6)$
Explanation
This linearization involves a fractional x-value with positive integer derivative. Using L(x) = J(a) + J'(a)(x - a) with a = 1/2, J(1/2) = 1, and J'(1/2) = 6. The linear approximation becomes L(x) = 1 + 6(x - 1/2). The tangent line passes through (1/2, 1) with slope 6, increasing rapidly by 6 units per unit increase in x. Linearizing at fractional points requires careful handling of the (x - a) term. Choice C uses -6 instead of +6, representing decreasing behavior when the positive derivative indicates increase. When linearizing at fractional points, maintain the exact fractional form in (x - a).
For $f$, $f(0)=1$ and $f'(0)=-frac12$. What is the linear approximation near $x=0$?
$L(x)=1+\tfrac12(x-0)$
$L(x)=1-\tfrac12(x-0)$
$L(x)=0-\tfrac12(x-1)$
$L(x)=-\tfrac12+1(x-0)$
$L(x)=1-0(x+\tfrac12)$
Explanation
This linearization problem involves fractional derivative and centers at x = 0. Using L(x) = f(a) + f'(a)(x - a) with a = 0, f(0) = 1, and f'(0) = -1/2. The linearization at the origin simplifies to L(x) = 1 + (-1/2)(x - 0) = 1 - (1/2)x. Since we're linearizing at x = 0, the (x - 0) term becomes just x. This tangent line starts at (0, 1) and has slope -1/2, decreasing as x increases. Choice C uses +1/2 instead of -1/2, giving an incorrect positive slope. When linearizing at x = 0, the formula reduces to f(0) + f'(0)x.
A function $M$ has $M(\pi)=2\pi$ and $M'(\pi)=2$. What is the linear approximation near $x=\pi$?
$L(x)=\pi+2(x-2\pi)$
$L(x)=2\pi+\pi(x-2)$
$L(x)=2\pi-2(x-\pi)$
$L(x)=2+2\pi(x-\pi)$
$L(x)=2\pi+2(x-\pi)$
Explanation
This linearization involves $\pi$ values with positive integer derivative. Using $L(x) = M(a) + M'(a)(x - a)$ with a = $\pi$, M($\pi$) = 2$\pi$, and M'($\pi$) = 2. The linear approximation becomes $L(x) = 2\pi + 2(x - \pi)$. The tangent line passes through ($\pi$, 2$\pi$) with slope 2, increasing by 2 units per unit increase in x. The function value is twice the linearization point, creating interesting geometric relationships. Choice C uses -2 instead of +2, representing decreasing behavior when the derivative indicates increase. When working with $\pi$ values, maintain exact forms and verify derivative signs for proper function behavior.