Find cos u/2 if tan u = -15/8 with an
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Find cos u/2 if tan u = -15/8 with an

[From: ] [author: ] [Date: 12-07-17] [Hit: ]
......
interval of 270 to 360 degrees

From my work which is listed below, I got an answer of 16/17 but the answer sheet says it is (34√34)/34. What am I doing that is so completely wrong?

Tan u = -15/8
hypotenuse= 8^2 + -15^2 = c^2
hypotenuse =17
cos u = 8/17
cos(u/2) = 2cos(u)
= 2(8/17)
=(16/17)

-
You didn't use a half-angle formula.

Determine the sign based on location.
cos u/2 = ± √((1 + cos u)/2)

270 < u < 360
135 < u/2 < 180 --> Q2

cos u/2 = - √((1 + 8/17)/2

cos u/2 = -√(25/34) = -5(√34)/34
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