Question Video: Evaluating a Trigonometric Function of the Sum of Two Angles given Their Cosine Functions and Quadrants | Nagwa Question Video: Evaluating a Trigonometric Function of the Sum of Two Angles given Their Cosine Functions and Quadrants | Nagwa

# Question Video: Evaluating a Trigonometric Function of the Sum of Two Angles given Their Cosine Functions and Quadrants Mathematics • Second Year of Secondary School

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Find cos (π΄ + π΅) given cos π΄ = 15/17 and cos π΅ = 5/13 where π΄ and π΅ are acute angles.

02:56

### Video Transcript

Find the cos of π΄ plus π΅ given the cos of π΄ equals 15 over 17 and the cos of π΅ equals five over 13, where π΄ and π΅ are acute angles.

When we see this cos of π΄ plus π΅, it should remind us of our angle sum identities. We know that the cos of π΄ plus π΅ is equal to the cos of π΄ times the cos of π΅ minus the sin of π΄ times the sin of π΅. We have enough information for the first term since weβve already been given the value of cos of π΄ and cos of π΅. So how should we go about finding the sin of π΄ and the sin of π΅ if we know the cosine ratios? They are acute angles. And therefore, one strategy to find sin π΄ and sin π΅ would be to make right-angled triangles with these proportions.

First, we can draw a right triangle with angle π΄. We know that the cosine relationship will be equal to the adjacent side length over the hypotenuse. And so we would label the adjacent side to π΄ 15 and the hypotenuse 17. To find the sine relationship, weβll need to know this opposite side length, and that means weβll need to use the Pythagorean theorem. Weβll let our unknown side be lowercase π. And then weβll have 17 squared equals π squared plus 15 squared, which will give us 289 is equal to π squared plus 225. To isolate π, we subtract 225 from both sides, and then we get 64 is equal to π squared. Taking the square root of both sides, we get π equal to eight.

Weβre only interested in the positive square root since weβre dealing with distance. If π equals eight, then the sin of angle π΄ will be equal to eight over 17. If we consider a second right-angled triangle with angle π΅, its adjacent side length is five and its hypotenuse is 13. We should recognize that this is a Pythagorean triple. Itβs a set of positive integers that occur in the ratio π squared plus π squared equals π squared. And because we know the hypotenuse is 13 and one of the sides is five, this is a five, 12, 13 triangle. And therefore, the adjacent side length will be 12 and the sin of angle π΅ is 12 over 13. And we can plug that value in. 15 over 17 times five over 13 is 75 over 221. Eight over 17 times 12 over 13 is 96 over 221, which makes the cos of π΄ plus π΅ equal to negative 21 over 221.

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