ACT Math : Trigonometry

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #3001 : Act Math

In the triangle below, \displaystyle AB = 2 units and \displaystyle AC = 6 units. What is \displaystyle \sin C?

 

Triangle

Possible Answers:

\displaystyle \frac{\sqrt 10}{2}

\displaystyle \sqrt10

\displaystyle 3\sqrt10

\displaystyle \frac{2}{\sqrt 10}

\displaystyle \frac{1}{\sqrt 10}

Correct answer:

\displaystyle \frac{1}{\sqrt 10}

Explanation:

Because \displaystyle \sin C =\frac{AB}{BC}, we need to first find the length of BC.

 

Using the Pythagorean theorem, 

\displaystyle AB^2+AC^2=BC^2

\displaystyle BC=\sqrt {AB^2+AC^2}

\displaystyle BC=\sqrt {6^2+2^2}

\displaystyle BC=\sqrt{36+4} = \sqrt {40} =\sqrt{4\cdot10}

\displaystyle BC=2\sqrt10.

 

\displaystyle \sin C =\frac{AB}{BC}=\frac{2}{2\sqrt10}=\frac{1}{\sqrt10}

Example Question #1 : How To Find The Sine Of A Missing Side

Sinx

What is the value of \displaystyle x in the triangle above? Round to the nearest hundredth.

Possible Answers:

\displaystyle 9.41

\displaystyle 15.84

\displaystyle 10.72

\displaystyle 0.77

\displaystyle 18.28

Correct answer:

\displaystyle 10.72

Explanation:

We know that the sine of an angle is:

\displaystyle \frac{opposite}{hypotenuse}

Therefore, we can write for this question:

\displaystyle sin(50)=\frac{x}{14}

This allows us to solve for \displaystyle x easily:

\displaystyle x=14sin(50)=10.72462220366569

Rounding, this gives us \displaystyle 10.72.

Example Question #41 : Sine

A right triangle has leg lengths \displaystyle 8 and \displaystyle 10. What is the sine of the angle opposite from the side of length \displaystyle 10?

Possible Answers:

\displaystyle \frac{4}{\sqrt{41}}{}

\displaystyle \frac{\sqrt{3}}{2}

\displaystyle \frac{4}{5}

\displaystyle \frac{5}{\sqrt{41}}{}

Correct answer:

\displaystyle \frac{5}{\sqrt{41}}{}

Explanation:

Using SOHCAHTOA, the sine of an angle is simply the length of the side opposite to it over the hypotenuse; however, we do not have the length of the hypotenuse yet.

Using the Pythagorean Theorem, we can solve for it:

\displaystyle 8^2+10^2=h^2{}

\displaystyle h=\sqrt{164}=2\sqrt{41}{}

So, the sine of this angle is:

\displaystyle \frac{10}{2\sqrt{41}} = \frac{5}{\sqrt{41}}{}

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

Simplify: (sinΘ + cosΘ)2

Possible Answers:

cos2Θ -1

None of the answers are correct

2sinΘcosΘ -1

1 + sin2Θ

1 + cos2Θ

Correct answer:

1 + sin2Θ

Explanation:

Using the foil method, multiply.  Simplify using the Pythagorean identity sin2Θ + cos2Θ = 1 and the double angle identity sin2Θ = 2sinΘcosΘ.

Example Question #91 : Trigonometry

For the triangle Using_sin_to_find_angle, find \displaystyle \angle A in degrees to the nearest integer 

 

Note: The triangle is not necessarily to scale

Possible Answers:

\displaystyle 52^{\circ}

None of the other answers

\displaystyle 58^{\circ}

\displaystyle 38^{\circ}

\displaystyle 32^{\circ}

Correct answer:

\displaystyle 38^{\circ}

Explanation:

To solve this equation, it is best to remember the mnemonic SOHCAHTOA which translates to Sin = Opposite / Hypotenuse, Cosine = Adjacent / Hypotenuse, and Tangent = Opposite / Adjacent. Looking at the problem statement, we are given the side opposite of the angle we are trying to find as well as the hypotenuse. Therefore, we will be using the SOH part of our mnemonic. Inserting our values, this becomes \displaystyle SIN(\angle A) = \frac{8}{13}. Then, we can write \displaystyle \angle A = SIN^{-1}\left ( \frac{8}{13} \right ). Solving this, we get \displaystyle \angle A = 38^{\circ}

Example Question #2 : How To Find An Angle With Sine

A fifteen foot ladder is leaned up against a twelve foot building, reaching the top of the building. What is the angle made between the ladder and the ground? Round to the nearest hundredth of a degree.

Possible Answers:

\displaystyle 37.14$^{\circ}$

\displaystyle 51.34$^{\circ}$

\displaystyle 41.99$^{\circ}$

\displaystyle 36.87$^{\circ}$

\displaystyle 53.13$^{\circ}$

Correct answer:

\displaystyle 53.13$^{\circ}$

Explanation:

You can draw your scenario using the following right triangle:

Theta1

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

 or \displaystyle 53.13$^{\circ}$.

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

Theta2

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

 

Possible Answers:

\displaystyle 27.84$^{\circ}$

\displaystyle 23.58$^{\circ}$

\displaystyle 66.42$^{\circ}$

\displaystyle 21.80$^{\circ}$

\displaystyle 20.11$^{\circ}$

Correct answer:

\displaystyle 23.58$^{\circ}$

Explanation:

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

 or \displaystyle 23.58$^{\circ}$.

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 0.6^{\circ}

\displaystyle 53.1^{\circ}

\displaystyle 31.0^{\circ}

\displaystyle 59.0^{\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 25.6^{\circ}

\displaystyle 0.4^{\circ}

\displaystyle 66.4^{\circ}

\displaystyle 68.2^{\circ}

\displaystyle 21.8^{\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

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.

 

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