jika sin teta - 2 cos teta =0, malka cos 2 teta= a.-4/5 b.-3/5 c.-1/2 d.1/2 e.4/5

sin T - 2 cos T = 0
sin T = 2cos T
sin T / cos T = 2
tan T = 2
tan T = 2/1, berarti y = 2 dan x = 1 ,karena tan T = y/x. dari Phitagoras r^2 = x^2 + y^2, maka
r= akar (2^2 + 1^2)
r = akar 5
jadi cos T = 1/akar 5
cos 2T = cos ( T + T)
= cos^2 T - sin^2 T
= cos^2 T - ( 1 - cos^2 T )
= 2cos^2 T - 1
= 2 (1/akar 5)^2 - 1
= 2.1/5 - 1
= 2/5 - 1
= 2/5 -5/5
= -3/5