1.
t=sinθ-cosθ
t²=(sinθ-cosθ)²=sin²θ+cos²θ-2sinθcosθ=1-sin(2θ)
sin(2θ)=1-t²
y=sin(2θ)+sinθ-cosθ=1-t²+t=-t²+t+1
t=sinθ-cosθ=√2sin(θ-π/4)
0≤θ≤π
-π/4≤θ-π/4≤3π/4
-√2/2≤sin((θ-π/4)≤1
-1≤√2sin((θ-π/4)≤√2
-1≤t≤√2
t的取值范围为[-1,√2].
(2)
y=-t²+t+1=-(t-1/2)²+5/4
当t=1/2时,y有最大值ymax=5/4
当t=-1时,y有最小值ymin=-1