1-x^2≥0-1≤x≤1设x=sina,-π/2≤a≤π/2则y=(sina)^2+cosa=1-(cosa)^2+cosa=-(cosa-1/2)^2+5/40≤cosa≤11≤y≤5/4 解法2令√(1-x^2)=t 0≤t≤1x^2=1-t^2y=1-t^2+t=-(t-1/2)^2+5/41≤y≤5/4