SOLUTION: If sinA=-3/5 with A in Q3 find cosA/2

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Question 1033520: If sinA=-3/5 with A in Q3 find cosA/2
Answer by ikleyn(52832) About Me  (Show Source):
You can put this solution on YOUR website!
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If sinA=-3/5 with A in Q3 find cosA/2
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First find  cos(A) = -sqrt%281-sin%5E2%28A%29%29 = -sqrt%281-%28-3%2F5%29%5E2%29 = -sqrt%281-9%2F25%29%29 = -sqrt%28%2825-9%29%2F25%29%29 = -sqrt%2816%2F25%29 = -4%2F5.


   The sign is "-" before the square root, since A is in Q3.


Now,  cos%28A%2F2%29 = -sqrt%28%281+-+cos%28A%29%29%2F2%29    (see the lesson Trigonometric functions of half argument in this site)


= -sqrt%28%281-%28-4%2F5%29%29%2F2%29 = -sqrt%289%2F10%29 = -3%2Fsqrt%2810%29 = %28-3%2Asqrt%2810%29%29%2F10.


   The sign is "-" because A%2F2 is in Q2 and cosine is negative in Q2.