ACT Math : Cosine

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Find The Cosine Of An Angle

What is cos θ?

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Possible Answers:

Correct answer:

Explanation:

cos = adjacent/hypotenuse = 

To get the radical out of the denominator, we multiply:

Example Question #121 : Trigonometry

What is the cosine of the angle formed between the -axis and the line passing through  with a slope of ?  Round to the nearest hundredth.

Possible Answers:

Correct answer:

Explanation:

You do not even need to compute the line for this question.  All that you need to do is note that you can make a little right triangle with a height of  and a base of , which you get from the slope of the line.  So, to compute the cosine, you will need to find the hypotenuse using the Pythagorean theorem:

So, our little triangle looks like:

Cos1

The cosine will be the adjacent side, , divided by the hypotenuse, :

, or approximately .

Example Question #3 : How To Find The Cosine Of An Angle

What is the  of an angle with ?

Possible Answers:

Correct answer:

Explanation:

Since the sine of the angle is , that means the opposite side of the triangle is  and the hypotenuse is . Automatically, you know this is a special 3-4-5 right triangle, and that the missing side is 3. If not, you could also find the 3rd side by doing Pythagorean Theorem. This gives you your answer of 

Example Question #4 : How To Find The Cosine Of An Angle

Right triangle  has sides  and . What is the cosine of 

Possible Answers:

Correct answer:

Explanation:

SOHCAHTOA tells us that , and we know the hypotenuse is the longest side of the triangle, . Our adjacent side will be the other side that has  as a vertex, .

Thus, .

Example Question #5 : How To Find The Cosine Of An Angle

Right triangle  has sides  and . What is the cosine of 

Possible Answers:

Correct answer:

Explanation:

SOHCAHTOA tells us that , and we know the hypotenuse is the longest side of the triangle, . Our adjacent side will be the other side that has  as a vertex, .

Thus, .

Example Question #1 : How To Find Positive Cosine

The value of a cosine is positive in which quadrants?

 

Possible Answers:

The 3rd only

The 4th only

The 1st and 3rd

The 1st and 4th

Correct answer:

The 1st and 4th

Explanation:

The cosine is positive in the 1st and 4th quadrants and negative in 2nd and 3rd

Example Question #1 : How To Find Positive Cosine

Which of the following is equal to ?

Possible Answers:

Correct answer:

Explanation:

Here, we use the SOHCAHTOA ratios and the fact that csc x = 1 / sin x.

Cosine x = adjacent side length / hypotenuse length

Cosecant x = 1 / sin x = hypotenuse / opposite

(Adjacent / hypotenuse) * (hypotenuse / opposite) = Adjacent / opposite = Cotangent x.

Example Question #2 : How To Find Positive Cosine

 and  is between  and .  What is the value of ?

Possible Answers:

Correct answer:

Explanation:

For  to , we know that .  So, the question asks, what is the value of , where .  Therefore, it is asking what the value of  is, which is .

Example Question #4 : How To Find Positive Cosine

To the nearest , what is the cosine formed from the origin to ? Assume counterclockwise rotation.

Possible Answers:

Correct answer:

Explanation:

If the point to be reached is , then we may envision a right triangle with sides  and , and hypotenuse . The Pythagorean Theorem tells us that , so we plug in and find that: 

Thus, 

Now, SOHCAHTOA tells us that , so we know that:

Thus, our cosine is approximately .

Example Question #5 : How To Find Positive Cosine

Two drivers race to a finish line. Driver A drives north  blocks, and east  blocks and crosses the goal. Driver B drives the shortest direct route between the two points. Relative to east, what is the cosine of the angle at which Driver B raced? Round to the nearest .

Possible Answers:

Correct answer:

Explanation:

If the point to be reached is  blocks north and  blocks east, then we may envision a right triangle with sides  and , and hypotenuse . The Pythagorean Theorem tells us that , so we plug in and find that: 

Thus, 

Now, SOHCAHTOA tells us that , so we know that:

Thus, our cosine is approximately .

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