find cos 20 if cos theta equal to 2√2/3

Question

find cos 20 if cos theta equal to 2√2/3

Answer 1

cos(ß)=(2√2)/3,,,,,want cos(2ß)

know cos(2x)=cos²(x)-sin²(x)=2cos²(x)-1

cos(2ß)=2{8/9)-1=7/9