(0.75-0.1x)(500+100x)=120
-50x+75x+375-10x²=120
25x-10x²+255=0
10x²-25x-255=0
2x²-5x-51=0
判别式=(-5)²-4*2*(-51)=433
x=(5±√433)/4
x=(5+√433)/4或x=(5-√433)/4
(0.75-0.25x)(500+34x)=120
-125x+25.5x+375-8.5x²=120
-99.5x-8.5x²+255=0
85x²+995x-2550=0
17x²+199x-510=0
判别式=199²-4*17*(-510)=74281
x=(-199±√74281)/34
x=(-199+√74281)/34或x=(-199-√74281)/34