ACT Math : How to find an angle with cosine

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Find An Angle With Cosine

Soh_cah_toa

In the above triangle, \(\displaystyle a = 15\) and \(\displaystyle h = 25\). Find \(\displaystyle \theta\).

Possible Answers:

\(\displaystyle 53.1^{\circ}\)

\(\displaystyle 0.6^{\circ}\)

\(\displaystyle 59.0^{\circ}\)

\(\displaystyle 31.0^{\circ}\)

\(\displaystyle 36.9^{\circ}\)

Correct answer:

\(\displaystyle 53.1^{\circ}\)

Explanation:

With right triangles, we can use SOH CAH TOA to solve for unknown side lengths and angles. For this problem, we are given the adjacent and hypotenuse sides of the triangle with relation to the angle. With this information, we can use the cosine function to find the angle.

\(\displaystyle \cos = \frac{adjacent}{hypotenuse}\)

\(\displaystyle \cos\left ( \theta\right ) = \frac{15}{25} = 0.6\)

\(\displaystyle \arccos\left ( 0.6\right ) = \theta\)

\(\displaystyle 53.1^{\circ}= \theta\)

 

Example Question #1 : How To Find An Angle With Cosine

Soh_cah_toa

For the above triangle, \(\displaystyle a = 12\) and \(\displaystyle h = 30\). Find \(\displaystyle \theta\).

Possible Answers:

\(\displaystyle 21.8^{\circ}\)

\(\displaystyle 0.4^{\circ}\)

\(\displaystyle 68.2^{\circ}\)

\(\displaystyle 66.4^{\circ}\)

\(\displaystyle 25.6^{\circ}\)

Correct answer:

\(\displaystyle 66.4^{\circ}\)

Explanation:

With right triangles, we can use SOH CAH TOA to solve for unknown side lengths and angles. For this problem, we are given the adjacent and hypotenuse sides of the triangle with relation to the angle. With this information, we can use the cosine function to find the angle.

\(\displaystyle \cos = \frac{adjacent}{hypotenuse}\)

\(\displaystyle \cos\left ( \theta\right ) = \frac{12}{30} = 0.4\)

\(\displaystyle \arccos\left ( 0.4\right ) = \theta\)

\(\displaystyle 66.4^{\circ}= \theta\)

 

Example Question #1 : How To Find An Angle With Cosine

Soh_cah_toa

For the above triangle, \(\displaystyle a = 17\) and \(\displaystyle h = 13\). Find \(\displaystyle \theta\).

Possible Answers:

This triangle cannot exist.

\(\displaystyle 49.9^{\circ}\)

\(\displaystyle 52.6^{\circ}\)

\(\displaystyle 37.4^{\circ}\)

\(\displaystyle 40.1^{\circ}\)

Correct answer:

This triangle cannot exist.

Explanation:

With right triangles, we can use SOH CAH TOA to solve for unknown side lengths and angles. For this problem, we are given the adjacent and hypotenuse sides of the triangle with relation to the angle. However, if we plug the given values into the formula for cosine, we get:

\(\displaystyle \cos = \frac{adjacent}{hypotenuse}\)

\(\displaystyle \cos\left ( \theta\right ) = \frac{17}{13} = 1.3\)

\(\displaystyle \arccos\left ( 1.3\right ) = \theta = \textup{no solution}\)

This problem does not have a solution. The sides of a right triangle must be shorter than the hypotenuse. A triangle with a side longer than the hypotenuse cannot exist. Similarly, the domain of the arccos function is \(\displaystyle \left [ -1, 1\right ]\). It is not defined at 1.3.

 

Example Question #1 : How To Find An Angle With Cosine

A \(\displaystyle 50\textup{-foot}\) rope is thrown down from a building to the ground and tied up at a distance of \(\displaystyle 27\textup{ feet}\)  from the base of the building. What is the angle measure between the rope and the ground? Round to the nearest hundredth of a degree.

Possible Answers:

\(\displaystyle 22.44$^{\circ}$\)

\(\displaystyle 32.68$^{\circ}$\)

\(\displaystyle 28.37$^{\circ}$\)

\(\displaystyle 57.32$^{\circ}$\)

\(\displaystyle 31.34$^{\circ}$\)

Correct answer:

\(\displaystyle 57.32$^{\circ}$\)

Explanation:

You can draw your scenario using the following right triangle:

Theta3

Recall that the cosine of an angle is equal to the ratio of the adjacent side to the hypotenuse of the triangle. You can solve for the angle by using an inverse cosine function:

\(\displaystyle \small \small \Theta = cos^-^1(\frac{27}{50})=57.31636115374206\) or \(\displaystyle \small 57.32\) degrees.

Example Question #1 : How To Find An Angle With Cosine

Theta6

What is the value of \(\displaystyle \small \Theta\) in the right triangle above? Round to the nearest hundredth of a degree.

Possible Answers:

\(\displaystyle 18.56$^{\circ}$\)

\(\displaystyle 70.38$^{\circ}$\)

\(\displaystyle 19.62$^{\circ}$\)

\(\displaystyle 71.44$^{\circ}$\)

\(\displaystyle 68.49$^{\circ}$\)

Correct answer:

\(\displaystyle 70.38$^{\circ}$\)

Explanation:

Recall that the cosine of an angle is equal to the ratio of the adjacent side to the hypotenuse of the triangle. You can solve for the angle by using an inverse cosine function:

\(\displaystyle \small \Theta = cos^-^1(\frac{47}{140})=70.38402086459453\) or \(\displaystyle 70.38$^{\circ}$\).

Example Question #1 : How To Find An Angle With Cosine

A support beam (buttress) lies against a building under construction. If the beam is \(\displaystyle 25\) feet long and strikes the building at a point \(\displaystyle 24\) feet up the wall, what angle does the beam strike the building at? Round to the nearest degree.

Possible Answers:

\(\displaystyle 16^{\circ}\)

\(\displaystyle 23^{\circ}\)

\(\displaystyle 18^{\circ}\)

\(\displaystyle 22^{\circ}\)

\(\displaystyle 7^{\circ}\)

Correct answer:

\(\displaystyle 16^{\circ}\)

Explanation:

Our answer lies in inverse functions. If the buttress is \(\displaystyle 25\) feet long and is \(\displaystyle 24\) feet up the ladder at the desired angle, then:

\(\displaystyle \textup{cos }x^{\circ} = \frac{24}{25}\)

Thus, using inverse functions we can say that \(\displaystyle x = \textup{cos}^{-1}\frac{24}{25} \approx 16.26\)

Thus, our buttress strikes the buliding at approximately a \(\displaystyle 16^{\circ}\) angle.

Example Question #1 : How To Find An Angle With Cosine

A stone monument stands as a tourist attraction. A tourist wants to catch the sun at just the right angle to "sit" on top of the pillar. The tourist lies down on the ground \(\displaystyle 8\) meters away from the monument, points the camera at the top of the monument, and the camera's display reads "DISTANCE -- \(\displaystyle 12\) METERS". To the nearest \(\displaystyle .01\) degree, what angle is the sun at relative to the horizon?

Possible Answers:

\(\displaystyle 78.44^{\circ}\)

\(\displaystyle 23.20^{\circ}\)

\(\displaystyle 48.19^{\circ}\)

\(\displaystyle 18.13^{\circ}\)

\(\displaystyle 62.82^{\circ}\)

Correct answer:

\(\displaystyle 48.19^{\circ}\)

Explanation:

Our answer lies in inverse functions. If the monument is \(\displaystyle 8\) meters away and the camera is \(\displaystyle 12\) meters from the monument's top at the desired angle, then:

\(\displaystyle \textup{cos} x^{\circ} = \frac{8}{12}\)

Thus, using inverse functions we can say that \(\displaystyle x = \textup{cos}^{-1}\frac{8}{12} \approx 48.189\)

Thus, our buttress strikes the buliding at approximately a \(\displaystyle 48.19^{\circ}\) angle.

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