## abbyl94 Group Title cos theta = 4/5, 0˚< theta < 90˚ use the information given to find sin2theta, cos2theta, and tan 2theta one year ago one year ago

1. satellite73 Group Title

you need $$\sin(\theta)$$ so compute these do you know how to find it?

2. abbyl94 Group Title

the identities?

3. satellite73 Group Title

|dw:1358823887705:dw|

4. alanli123 Group Title

sin 2 theta = 2 sin theta cos theta cos 2 theta = 2 cos ^2 theta - 1 tan 2 theta = sin 2 theta / cos 2 theta

5. abbyl94 Group Title

do i plug in 4/5 where theta is?

6. Azteck Group Title

If costheta=4/5 then sintheta= 3/5 $\sin2\theta=2\sin\theta \cos\theta$

7. satellite73 Group Title

there is a picture of an angle whose cosine is $$\frac{4}{5}$$ you find the third side by pythagoras, or recalling the 3 - 4 - 5 right triangle this tells you $$\sin(\theta)=\frac{3}{5}$$

8. abbyl94 Group Title

oh okay

9. abbyl94 Group Title

so 2 sin theta = 2 sin 3/5 cos 4/5

10. satellite73 Group Title

no do not make that mistake, it is a common one do not use $$\theta=\frac{4}{5}$$ use $$\cos(\theta)=\frac{4}{5}$$ so $\sin(2\theta)=2\times \frac{4}{5}\times \frac{3}{5}$

11. satellite73 Group Title

no not the cosine and sine of those numbers, those numbers ARE the cosine and sine

12. satellite73 Group Title

use the numbers themselves, they are your sines and cosines

13. abbyl94 Group Title

okay thank you!

14. satellite73 Group Title