if cos x = 2 then

Question

what is cos2x?

Answer 1

Cos2x = 2cos^2x - 1                   [Since Cos2u = 2cos^2u - 1]

           = 2(cosx)^2 - 1

           = 2(2) - 1

           = 4 - 1

           = 3

Therefore if cosx=2 then Cos2x = 3.

Comments

The value of cos(2x) = 7.

Answer 2

The function cos x = 2.

cos(2x)

= (2 cos²(x) - 1)

= { 2[ (cos(x) ]² - 1 }

Substitute cos(x) = 2

= { 2[2]² - 1 }

= { 2[4] - 1 }

= {8 - 1}

= 7.

The value of cos(2x) = 7