## rubyhunter 3 years ago if cos x = 4/11 find cos 2A (HELP PLEASE)

1. maym0Re97

by 2A do you mean 2x?

2. maym0Re97

If that's the case, then your'e going to do arc cos of 4/11, which yields x, then plug that value of x into cos2x

3. rubyhunter

i dnt get ti can you walk me through it

4. maym0Re97

ok so by 2A did you actually mean cos2x?

5. rubyhunter

yes

6. shubhamsrg

why dont you simply use this gem of a formulla : cos 2x = (2cos^2 x) - 1 hope this helps..

7. rubyhunter

no it doesnt help CAUSE I DNT KNOW HOW TO DO IT :(

8. shubhamsrg

9. maym0Re97

Ok so use your graphing calculator and there should be a button that's like cos, and right above it you'll see cos^-1. Plug into your calcultar, cos^-1(4/11), because what that's saying is cos of the whatever that is, is going to be 4/11. Now you have x. Now plug whatever you got for x back into your calculator.

10. maym0Re97

Back into the equation cos2x.

11. rubyhunter

wait y am i doin cos-1 instead of cos

12. maym0Re97

because that's how you're going to find x. It's just like how if you had x^2=4, you'd find the square root of both sides. In this case you're getting rid of the cos.

13. rubyhunter

so 68,67631374

14. rubyhunter

what do i do after

15. maym0Re97

ok, so is that what you're getting for cos^-1(4/11)?

16. rubyhunter

yea

17. maym0Re97

ok you have that value for x, now plug that in for x in cos2x. So plug in cos(2*68.67). That SHOULD be how you do it, I haven't done stuff like this in two years.

18. rubyhunter

so is it a -.7353878608

19. maym0Re97

Yeah uhhhh that should be it, apologies if it isn't though, I don't have a calculator with me.

20. jishan

use to apply cos2x=cos^2x-sin^2x cox=4/11 sinx=squareroot( 1-cos^2x)