y=x+√(1-x^2)
1-x^2≥0
x^2≤1
-1≤x≤1
令x=sint,则
y=sint+cost
=√2sin(t+π/4)
当t=π/4时,y=√2最大,此时x=√2/2
当x=-1时,y=-1最小
-1≤y≤√2