Question

If cos2 x + 2cosx - 2 = 0, then cosx = _____.

-1 ±√3
1 ±√3
±√3
1

Answer 1

Can you solve?

\(\large\color{black}{ \displaystyle k^2+2k-2=0 }\)

Answer 2

You can do this:

\(\large\color{black}{ \displaystyle k^2+2k-2=0 }\)
\(\large\color{black}{ \displaystyle k^2+2k-2\color{red}{+3}=0 \color{red}{+3} }\)
\(\large\color{black}{ \displaystyle k^2+2k+1=3 }\)
\(\large\color{black}{ \displaystyle \left(k+1\right)^2=3 }\)

Answer 3

via «completing the square» method.