（100-2x）(50-2x)＝3600 把它乘出来...变成5000-200x-100x＋4(x的

(100-2x)(50-2x)=[100+(-2x)][50+(-2x)]=100*50+(-2x)*50+100*(-2x)+(-2x)*(-2x)
=5000+(-100x)+(-200x)+4x^2=5000+(-300x)+4x^2=5000-300x+4x^2
or
2(50-x)*2(25-x)=4*900 =>(50-x)(25-x)=900 =>1250-75x+x^2-900=0
=> x^2-75x+350=0 =>(x-70)(x-5)=0 => x=70 or 5