Posted by **shandra** on Monday, January 2, 2012 at 10:04pm.

Given tan theta=-8/5 and sin theta < 0, find cos theta.

- Math-pre-calculus -
**Reiny**, Monday, January 2, 2012 at 10:40pm
if tanŲ < 0 and sinŲ < 0 , then Ų must be in quad IV

from tanŲ = -8/5 in IV, we have x = 5, y = -8

r^2 = 25+64 = 89

r = √89

then cosŲ = 5/√89

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