SOLUTION: Given 90° < a < 180° and sin(a) = c/d, find the value for cos(a) in terms of c and/or d.

Algebra ->  Trigonometry-basics -> SOLUTION: Given 90° < a < 180° and sin(a) = c/d, find the value for cos(a) in terms of c and/or d.       Log On


   

Question 1194581: Given 90° < a < 180° and sin(a) = c/d, find the value for cos(a) in terms of c and/or d.
Answer by ikleyn(52908) About Me  (Show Source):
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Given 90° < a < 180° and sin(a) = c/d, find the value for cos(a) in terms of c and/or d.
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You are given that angle "a" is in the second quadrant, where cosine function is negative.


THEREFORE,  cos(a) = -sqrt%281-sin%5E2%28a%29%29 = -sqrt%281-%28c%2Fd%29%5E2%29.    ANSWER


Notice the sign  " - "  (minus)  before the square root: it is because cosine is negative in QII,
as it is stated/explained above.

Solved and explained.