(x/4)²+[(20-x)/4]²=x²/16+(400-40x+x²)/16=(x²+400-40x+x²)/16=(2x²-40x+400)/16=(x²-20x+200)/8=(x²-20x+100+100)/8=[(x-10)²+100]/8=(x-10)²/8+25/2当x=10时面积之和最小。最小值为25/2。
