Question 744796
Ok, i finally get the answer to my question. the answer is 16 cm. Im running out of time in writing my solution, so may be next time i will show it. By the way i break the trapezoid into to 2 right triangles and 1 rectangle and add its areas, then i came up with this equation {{{sqrt(-x^4-16x^3+1024x+4096)}}} then to get the maximum area, derive with respect to x and get the roots of x, the answer is 4 and -8 so the 4th sides is {{{2x+8=16}}}