Question

if cos x = 4/11 find cos 2A (HELP PLEASE)

Answer 1

by 2A do you mean 2x?

Answer 2

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

Answer 3

i dnt get ti can you walk me through it

Answer 4

ok so by 2A did you actually mean cos2x?

Answer 5

yes

Answer 6

why dont you simply use this gem of a formulla :

cos 2x = (2cos^2 x) - 1

hope this helps..

Answer 7

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

Answer 8

nevermind,,i cant help you further then :/

Answer 9

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.

Answer 10

Back into the equation cos2x.

Answer 11

wait y am i doin cos-1 instead of cos

Answer 12

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.

Answer 13

so 68,67631374

Answer 14

what do i do after

Answer 15

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

Answer 16

yea

Answer 17

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.

Answer 18

so is it a -.7353878608

Answer 19

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

Answer 20

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