一元二次方程
(350-10a)*(a-21)=400 要过程!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!把每一步都写出来
参考答案:(350-10a)*(a-21)=400
350a-10a^2-350*21+210a=400
10a^2-560a+7750=0
10(a-28)^2=90
a=28+3ora=28-3
a=31or25