anonymous
  • anonymous
if cos x = 4/11 find cos 2A (HELP PLEASE)
Trigonometry
  • Stacey Warren - Expert brainly.com
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SOLVED
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jamiebookeater
  • jamiebookeater
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anonymous
  • anonymous
by 2A do you mean 2x?
anonymous
  • anonymous
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
anonymous
  • anonymous
i dnt get ti can you walk me through it

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anonymous
  • anonymous
ok so by 2A did you actually mean cos2x?
anonymous
  • anonymous
yes
shubhamsrg
  • shubhamsrg
why dont you simply use this gem of a formulla : cos 2x = (2cos^2 x) - 1 hope this helps..
anonymous
  • anonymous
no it doesnt help CAUSE I DNT KNOW HOW TO DO IT :(
shubhamsrg
  • shubhamsrg
nevermind,,i cant help you further then :/
anonymous
  • anonymous
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.
anonymous
  • anonymous
Back into the equation cos2x.
anonymous
  • anonymous
wait y am i doin cos-1 instead of cos
anonymous
  • anonymous
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.
anonymous
  • anonymous
so 68,67631374
anonymous
  • anonymous
what do i do after
anonymous
  • anonymous
ok, so is that what you're getting for cos^-1(4/11)?
anonymous
  • anonymous
yea
anonymous
  • anonymous
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.
anonymous
  • anonymous
so is it a -.7353878608
anonymous
  • anonymous
Yeah uhhhh that should be it, apologies if it isn't though, I don't have a calculator with me.
anonymous
  • anonymous
use to apply cos2x=cos^2x-sin^2x cox=4/11 sinx=squareroot( 1-cos^2x)

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