Chứng minh đẳng thức: a) cos5xcos3x + sin7xsinx = cos2xcos4x. b) sin5x - 2sinx(cos2x + cos4x) = sinx. c) (sinx - siny)^2 + (cosx - cosy)^2 = 4sin^2x - y/2. d) tan3x - tan2x - tanx = tan3xtan2xtanx

a) cos 5x. cos 3x + sin7x.sinx=cos2x.cos4x
b) sin5x - 2sinx( cos2x + cos4x) = sinx
c) ( sinx - siny)^2 + (cosx - cosy)^2=4sin^2. x-y/2
d) tan3x - tan2x -tanx=tan3x.tn2x.tanx
e) 4tanx( 1-tan bìnhx)/(1+tan bình x)^2= sin 4x
f) 1+tan mũ 4x/ tan bình x+cot bình x=tan bình x
g) (tan2x - tanx)(sin2x-tanx)=tan bình x
h) sin x + cosx - sin(x-π/6) + cos( x--π/6)= √ 6 cos(x-π/12)
i) sinx.sin(π/3 -x)sin(π/3 +x)= 1/4 sin 3x
j) sin bình x +sin bình(π/3 -x) + sinx.sin(π/3 -x)=3/4