Question

Find all solutions (to three decimal places) of the equation in the given interval. ?????????

cos^2 x - 2cos x - 1= 0 [0,π]

Answer 1

remember double angle identity 2cos x - 1= cos(2A)

Answer 2

cos(2A) = cos^2-sin^2

Answer 3

cos^2 -cos^2 -sin^2 =0

Answer 4

can you continue?

Answer 5

uhh...im trying to find solutions, not solving a proof...

Answer 6

how about let y = cos x
then your equation is
y^2 - 2y - 1 =0

find the solutions for y
as a first step.

what do you get ?

Answer 7

use the quadratic formula
http://www.purplemath.com/modules/solvquad4.htm

Answer 8

I already got it.