Posted by **Jenny** on Saturday, June 19, 2010 at 5:10am.

If sinA = 1/3 and cosA<0

find the exact value of tan2A

- Maths 3u trig -
**Damon**, Saturday, June 19, 2010 at 6:35am
sinA = 1/3 and cosA<0

find the exact value of tan2A

What quadrant is A? Sin is +, cos is - so quadrant 2, so 2 A is quadrant 3 or 4

cos^2A = 1 - sin^2A = 1-1/9 = 8/9

cos A = -(2/3)sqrt 2

tanA = (1/3)/[-(2/3)sqrt2)] = -1/(2 sqrt 2)

tan 2A = 2 [-1/(2 sqrt 2)]/[1- 1/8]

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