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confused with trig simplifying watch

1. find sin2a when cosa = 3/4 and sin a = - root7 /4

ok so sin2A= 2 X - root7/4 X 3/4

so i got = -2root7 X 3 / 8 = -6root7 / 8

but it shows the answer as = -3root7/8 ??
2. 4*4 is 16.
Cancel the 2.
3. 4 times 4 is 16, not 8.
4. ahh, stupid things like this will make me fail my exam =p

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Updated: November 11, 2008
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