Answer:
cos2x=cox^2 x-1
Step-by-step explanation:
1.If cosx=0 then equality is established
2.If cosx doesn't equal 0 then:
cos(2x+x)=4cos^3 x-3cos x
cos2x*cosx-sin2x*sinx=4cos^2 x-3cosx
cos2x-2sin^2 x=4cos^2 x-3
cos2x=2+2cos^2 x-3
cos2x=cox^2 x-1
equality is established
