Posted by **Melanie** on Sunday, July 29, 2012 at 9:09pm.

suppose that sin0=-5/13 and that 0 is a quadrant III angle. what is the exact value of cos0

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**drwls**, Sunday, July 29, 2012 at 9:21pm
The "reference angle" that theta makes with the -x axis is sin^-1(5/13) . The cosine of that angle is 12/13.

(Think of a 5:12:13 right triangle)

Cosine is negative in the third quadrant.

The answer is -12/13

- math -
**Reiny**, Sunday, July 29, 2012 at 9:22pm
make a sketch of the right-angled triangle in quad III, with hypotenuse 13 and opposite 5.

By Pythagoras the adjacent would be 12 (the 5-12-13 triangle)

so cosØ = -12/13 ( by CAST, in III the cosine is negative)

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