(Headbang) 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?(Wondering)

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- Jul 20th 2009, 08:00 AMVikyneed help with homework
(Headbang) 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?(Wondering) - Jul 20th 2009, 08:10 AMyeongil
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.

01 - Jul 20th 2009, 08:20 AMViky
could you give me an example for one of them?

- Jul 20th 2009, 08:23 AMyeongil
$\displaystyle \begin{aligned}

\cos 160^{\circ} &= \cos (70^{\circ} + 90^{\circ}) \\

&= -\sin 70^{\circ} \\

&= -t

\end{aligned}$

01 - Jul 20th 2009, 08:24 AMViky
wow i don't know why i confused myself with this stuff. Thanks alot i understand now

- Jul 20th 2009, 08:34 AMyeongil
EDIT: I am sleep deprived. Sigh.