左边展开是
40*20+40*2x-20x-2x*x=1200
-2x^2+60x+800=1200
-2x^2+60x-400=0
x^2-30x+200 = 0
1 -20
1 -10
所以, (x-20)(x-10) = 0
则x=20, x=10
左边展开是
40*20+40*2x-20x-2x*x=1200
-2x^2+60x+800=1200
-2x^2+60x-400=0
x^2-30x+200 = 0
1 -20
1 -10
所以, (x-20)(x-10) = 0
则x=20, x=10