f(x)=2根号3sinxcosx+2cos²x-1
问题描述：
f(x)=2根号3sinxcosx+2cos²x-1
①f（x1）=6/5,x1∈[π/4,π/2],求cos2x1
答
f(x)=2√3sinxcosx+2cos²x-1
=√3sin(2x)+cos(2x)
=2[(√3/2)sin(2x)+(1/2)cos(2x)]
=2sin(2x+π/6)
x1∈[π/4,π/2]
2π/3≤2x1+π/6≤7π/6 cos(2x1+π/6)