## walter9906 2 years ago Find cos 2A, Sin A = -2/9

1. genius12

You will need to use some identities here:$\bf \cos(2a)=1-2\sin^2(a)$We know what sin(a) is, so we just plug that in to the identity and evaluate.$\bf \cos(2a)=1-2(-2/9)^2=..?$Can you evaluate that? @walter9906

2. walter9906

Thank you. I have one more. find Cos 2A $\cos A=\sqrt{3}/4$

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