SAT Math : Variables

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #1 : How To Use The Direct Variation Formula

and are the radius and volume, respectively, of a given sphere.

.

Which of the following is a true statement?

Possible Answers:

varies inversely as the sixth root of .

varies inversely as the sixth power of .

varies directly as .

varies directly as the sixth root of .

varies directly as the sixth power of .

Correct answer:

varies directly as the sixth power of .

Explanation:

The volume of a sphere can be calculated from its radius as follows:

Therefore, squaring both sides, we get

 

Substituting:

If we let the constant of variation be , we see that

,

and  varies directly as , the sixth power of .

 

Example Question #1 : Direct And Inverse Variation

The temperature at the surface of the ocean is . At  meters below the surface, the ocean temperature is . By how much does the temperature decrease for every  meters below the ocean's surface?

Possible Answers:

Correct answer:

Explanation:

This may seem confusing, but is pretty straightforward.

 

Thus, for every 125 meters below the surface, the temperature decreases by one degree.

To find how much it decreases with every 100 meters, we need to do the following:

Thus, the temperature decreases by  every 100 meters.

Example Question #2 : Direct And Inverse Variation

The square of  varies inversely with the cube of . If  when , then what is the value of  when

Possible Answers:

Correct answer:

Explanation:

When two quantities vary inversely, their products are always equal to a constant, which we can call k. If the square of x and the cube of y vary inversely, this means that the product of the square of x and the cube of y will equal k. We can represent the square of x as x2 and the cube of y as y3. Now, we can write the equation for inverse variation.

x2y3 = k

We are told that when x = 8, y = 8. We can substitute these values into our equation for inverse variation and then solve for k.

82(83) = k

k = 82(83)

Because this will probably be a large number, it might help just to keep it in exponent form. Let's apply the property of exponents which says that abac = ab+c.

k = 82(83) = 82+3 = 85.

Next, we must find the value of y when x = 1. Let's use our equation for inverse variation equation, substituting 85 in for k.

x2y3 = 85

(1)2y3 = 85

y3 = 85

In order to solve this, we will have to take a cube root. Thus, it will help to rewrite 8 as the cube of 2, or 23.

y3 = (23)5

We can now apply the property of exponents that states that (ab)c = abc.

y3 = 23•5 = 215 

In order to get y by itself, we will have the raise each side of the equation to the 1/3 power.

(y3)(1/3) = (215)(1/3)

Once again, let's apply the property (ab)c = abc.

y(3 • 1/3) = 2(15 • 1/3)

y = 25 = 32

The answer is 32. 

Example Question #1 : How To Use The Inverse Variation Formula

varies directly as and inversely as .

and .

Which of the following is true about ?

Possible Answers:

varies inversely as the square root of and directly as the fourth root of .

varies inversely as the square of and directly as the fourth power of .

varies directly as the square of and inversely as the fourth power of .

varies directly as both the square root of and the fourth root of .

varies directly as the square root of and inversely as the fourth root of .

Correct answer:

varies inversely as the square of and directly as the fourth power of .

Explanation:

varies directly as and inversely as , so for some constant of variation ,

.

We can square both sides to obtain:

.

, so .

By substitution,

.

Using as the constant of variation, we see that varies inversely as the square of and  directly as the fourth power of .

 

Example Question #1 : Direct And Inverse Variation

The radius of the base of a cylinder is ; the height of the same cylinder is ; the cylinder has volume 1,000.

Which of the following is a true statement?

Assume all quantities are positive.

Possible Answers:

varies inversely as the square root of .

varies inversely as the square of .

varies inversely as .

varies directly as the square of .

varies dorectly as the square root of .

Correct answer:

varies inversely as .

Explanation:

The volume of a cylinder can be calculated from its height and the radius of its base using the formula:

, so ;

, so .

The volume is 1,000, and by substitution, using the other equations:

If we take as the constant of variation, we get

,

meaning that varies inversely as .

 

 

Example Question #1 : How To Use The Inverse Variation Formula

varies directly as the square of and the cube root of , and inversely as the fourth root of . Which of the following is a true statement?

Possible Answers:

varies directly as the square root of and the eighth root of , and inversely as the sixth power of .

 varies directly as the square root of , and inversely as the eighth power of  and the sixth power of .

varies directly as the square root of and the eighth root of , and inversely as the sixth root of .

varies directly as the square root of and the eighth power of , and inversely as the sixth root of .

varies directly as the square root of , and inversely as the eighth root of and the sixth root of .

Correct answer:

varies directly as the square root of and the eighth root of , and inversely as the sixth root of .

Explanation:

varies directly as the square of and the cube root of , and inversely as the fourth root of , so, for some constant of variation ,

We take the reciprocal of both sides, then extract the square root:

Taking as the constant of variation, we see that varies directly as the square root of and the eighth root of , and inversely as the sixth root of .

Example Question #1 : How To Use The Inverse Variation Formula

If  varies inversely as , and  when , find  when .

Possible Answers:

Correct answer:

Explanation:

The formula for inverse variation is as follows: 

Use the x and y values from the first part of the sentence to find k. 

Then use that k value and the given x value to find y.

 

Example Question #1 : How To Find The Degree Of A Polynomial

Find the degree of the polynomial:

Possible Answers:

Correct answer:

Explanation:

To find the degree of a polynomial we must find the largest exponent in the function.

The degree of the polynomial  is 5, as the largest exponent of  is 5 in the second term. 

Example Question #2 : How To Find The Degree Of A Polynomial

What is the degree of the polynomial ?

Possible Answers:

Correct answer:

Explanation:

When a polynomial has more than one variable, we need to find the degree by adding the exponents of each variable in each term. 

 has a degree of 4 (since both exponents add up to 4), so the polynomial has a degree of 4 as this term has the highest degree. 

Example Question #2 : Polynomials

Find the degree of the following polynomial:

Possible Answers:

Correct answer:

Explanation:

When a polynomial has more than one variable, we need to find the degree by adding the exponents of each variable in each term.

Even though  has a degree of 5, it is not the highest degree in the polynomial - 

 has a degree of 6 (with exponents 1, 2, and 3). Therefore, the degree of the polynomial is 6.  

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