if sin 70=t write the following in terms of t:
1] cos 160
2] cos 70
3] sin 140
you need to use the expansion method right?
For #1, use the identity $\displaystyle \cos (x + 90^{\circ}) = -\sin x$.
For #2, use the Pythagorean identity $\displaystyle \sin^2 x + \cos^2 x = 1$.
For #3, use the double-angle identity $\displaystyle \sin (2x) = 2\sin x \cos x$.
In all three cases, substitute 70° for x.
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