Question

已知cos(π/4+x)=4/5,x∈(-π/2,-π/4).求（sin2x-2sin²x)/(1+tanx)

Answer 1

cos(π/4+X)=4/5,X∈(-π/2,-π/4),则π/4+X∈(-π/4,0),2X∈(-π,-π/2)sin（π/4+X）=-3/5,sin（π/2+2X）=2 sin（π/4+X）cos(π/4+X)=-24/25cos2X =sin（π/2+2X）=sin（π/2-2X）=-24/25,sin2X=-7/25（...