need help with homework

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July 20th 2009, 08:00 AM
Viky

need 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)
July 20th 2009, 08:10 AM
yeongil

For #1, use the identity $\cos (x + 90^{\circ}) = -\sin x$ .

For #2, use the Pythagorean identity $\sin^2 x + \cos^2 x = 1$ .

For #3, use the double-angle identity $\sin (2x) = 2\sin x \cos x$ .

In all three cases, substitute 70° for x.

01
July 20th 2009, 08:20 AM
Viky

could you give me an example for one of them?
July 20th 2009, 08:23 AM
yeongil

$\begin{aligned}<br /> \cos 160^{\circ} &= \cos (70^{\circ} + 90^{\circ}) \\<br /> &= -\sin 70^{\circ} \\<br /> &= -t<br /> \end{aligned}$

01
July 20th 2009, 08:24 AM
Viky

wow i don't know why i confused myself with this stuff. Thanks alot i understand now
July 20th 2009, 08:34 AM
yeongil

EDIT: I am sleep deprived. Sigh.

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