The side length of square ABCD is 20, CD is extended to e, connecting be. If the area of triangle ABF is 30 larger than that of triangle def, the length of De is calculated

From the fact that the area of triangle ABF is 30 larger than that of triangle def, we can think that the area of square ABCD is 30 larger than that of triangle BCE. The area of square is 20 * 20 = 400, so the area of triangle BCE is 400-30 = 370, because the height of triangle = area * 2 / bottom, so EC = 370 * 2 / 20 = 37, de = 37-20 = 17

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