# Cosine Problem

• May 9th 2012, 11:54 PM
Chuka
[SOLVED] Cosine Problem
Find the small positive number x such that

cos(x^2+2x+9)=0

So far I've tried this equation: x^2+2x+(9-(2n+1)pi/2)=0
put it into quadratic and for that i got x=-4.8688x10^-4 which wasn't correct

Also tried: x^2+2x+(9-pi/2)=0
got 1.9033 which also wasn't correct

T_T;; please help I only have 1 more chance or I won't get full credit. I am totally lost, I have NO IDEA what to do...but I did try.
• May 10th 2012, 12:33 AM
Goku
Re: Cosine Problem
Quote:

Originally Posted by Chuka

Also tried: x^2+2x+(9-pi/2)=0
got 1.9033 which also wasn't correct
.

How did you get 1.9033,
solutions to the above equation is :
x1 = -1 + 2.5356i
x2 = -1 + 2.5356 i

In other words these give you no solutions,

Try to solve this quadratic equation:
x^2+2x+(9-7pi/2)=0 // cos((7 * pi) / 2) = 0

and remember that for the quadratic equation to give you real values:

(2n+1)pi/2 > 9 // solve for what values of n you get real values....
• May 10th 2012, 01:20 AM
Chuka
Re: Cosine Problem
OMG THANK YOU!!!!!!!! Thank god XD, got it on my last chance.
I actually did try 7pi/2 before but I made errors. Fail T_T..