# need help with homework

• Jul 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)
• Jul 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
• Jul 20th 2009, 08:20 AM
Viky
could you give me an example for one of them?
• Jul 20th 2009, 08:23 AM
yeongil
\begin{aligned}
\cos 160^{\circ} &= \cos (70^{\circ} + 90^{\circ}) \\
&= -\sin 70^{\circ} \\
&= -t
\end{aligned}

01
• Jul 20th 2009, 08:24 AM
Viky
wow i don't know why i confused myself with this stuff. Thanks alot i understand now
• Jul 20th 2009, 08:34 AM
yeongil
EDIT: I am sleep deprived. Sigh.