初二
不解方程{2x+3y=49/8 {x-2y=10/7 求7y(x-2y)^2-2(2y-x)^3的值.
7y(x-2y)^2-2(2y-x)^3----x-2y=10/7 =7y*100/49-2(100/49)(2y-x) =100/49(7y-4y+x) =100/49(3y+x)-------------2x+3y=49/8 =100/49*(49/8) =100/8=25/2
7y(x-2y)^2-2(2y-x)^3 =(x-2y)^2*[7y-(2y-x)] =(x-2y)^2*(x+7y) =(x-2y)^2*[(2x+3y)-(x-2y)] =(10/7)^2*(49/8-10/7) =100/49*(343-80)/7 =100*263/(49*7) =26300/343.
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