- #1
BOAS
- 552
- 19
Homework Statement
Solve sin2θ - 1 = cos2θ in range 0 ≤ θ ≤ 360°
Homework Equations
The Attempt at a Solution
I always struggle with the end of these questions, deciding which answers are correct.
Here's what I have done;
let 2θ = x
cosx + 1 = sinx
cos2x + sin2x = 1
sin x = √(1 - cos2x)
cosx + 1 = √(1 - cos2x)
square both sides
cos2x + 2cosx + 1 = 1 - cos2x
2cos2x + 2cosx = 0
factor out 2cosx
2cosx (cosx + 1) = 0
cosx = 0 when x = 270, 90
so θ = 135, 45
cosx - 1 = 0
x = 180, 270
θ = 90, 135
Only 90 and 45 are in the first quadrant where cos is +ve, does that mean these are the answers I want?
Thanks for any help you can give, I hope my question is clear.