(3-x)(4-x)=48-20x+2x²
(x-3)(x-4)=48-20x+2x²
x²-7x+12=48-20x+2x²
x²-7x+12-48+20x-2x²=0
-x²+13x-36=0
x²-13x+36=0
（x-4）（x-9）=0
x-4=0或x-9=0
所以x1=4或x2=9.
(3-x)(4-x)=48-20x+2x²怎么解
(3-x)(4-x)=48-20x+2x²
(x-3)(x-4)=48-20x+2x²
x²-7x+12=48-20x+2x²
x²-7x+12-48+20x-2x²=0
-x²+13x-36=0
x²-13x+36=0
（x-4）（x-9）=0
x-4=0或x-9=0
所以x1=4或x2=9.