diketahui tan alfa=p. tentukan cos 2 alfa

tan A = p, tan = y/x.berarti y = p dan x = 1
r = akar( 1+p^2)
cos A = 1/V(1+p^2)
cos 2A = 2cos^2 A - 1
= 2 { 1/V(1+p^2)^2 - 1 -
= 2 ( 1/(1+p^2) - 1
= 2 / (1 + p^2) -1
= { 2 -1(1+p^2) }/ (1+p^2)
=( 1 - p^2)/(1+p^2)