① The area of the triangle is:12x (60-x) =-12x2+30x =-12 (x-30) 2+450.

When x=30, the maximum area of the triangle is 450cm2；;

② The area of the rectangle is: x (60-2x) =-2x2+60x =-2 (x-15) 2+450,

When x= 15, the maximum area of the rectangle is 450cm2；;

③ The area of isosceles trapezoid is 32x (60-2x)+2x12x12x × 32x =-334x2+303x =-334 (x-20) 2+3003.

When x=20, the maximum area of isosceles trapezoid is 300.