设t=sinx+cosx=(√2）sain(x+π/4)(0<=x<=π）,则
π/4<=x+π/4<=5π/4,
t∈[-1,√2],
sin2x=t^2-1,
∴y=t^2-8t+18=(t-4)^2+2,↓,
t=√2时y取最小值20-8√2,
t=-1时y取最大值27.