# Thread: trig equations help?

1. ## trig equations help?

I'm stuck on these questions:

1. Find theta in the interval 0 <theta<360 that satisfies the equation $cos^2theta=sin^2theta+1$

2. $2cos^2x+2cosx-1=0$

2. Hello,
Originally Posted by eh501
I'm stuck on these questions:

1. Find theta in the interval 0 <theta<360 that satisfies the equation $cos^2theta=sin^2theta+1$
recall the identity cos²x+sin²x=1.

hence your equation becomes :
$1-\sin^2 \theta=\sin^2 \theta+1$
which simplifies to :
$2 \sin^2 \theta=0$
$\sin \theta=0$

2. $2cos^2x+2cosx-1=0$
Let $X=\cos(x)$
Solve the quadratic
and then find the solutions ^^