Posted by **steve** on Wednesday, July 14, 2010 at 9:20am.

can you help me find cos 2x if sin x = -12/13 (x in 3rd quadrant)

- trig -
**Henry**, Thursday, July 15, 2010 at 5:06pm
GIVEN: SinX = -12 / 13

SinX = 12/13 = 0.9231, X=67.38(1st.quad.), X = 67.38 + 180 = 247.38

( 3rd. quad, ), Z

2X = 2*247.34 = 494.76

COS2X = COS 494.76 = -0.7041

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