f(x)=(sinx+cosx)²+2cos²x
=sin²x+2sinxcosx+cos²x+2cos²x
=1+2sinxcosx+2cos²x
=2sincosx+2cos²x-1+2
=sin2x+cos2x+2.正弦,余弦二倍角公式
=√2sin(2x+π/4)+2.辅助角公式
令π/2+2kπ
f(x)=(sinx+cosx)²+2cos²x
=sin²x+2sinxcosx+cos²x+2cos²x
=1+2sinxcosx+2cos²x
=2sincosx+2cos²x-1+2
=sin2x+cos2x+2.正弦,余弦二倍角公式
=√2sin(2x+π/4)+2.辅助角公式
令π/2+2kπ