Question

初二数学题 一元一次不等式于一次函数

http://wenwen.soso.com/z/q209057037.htm
题目第三小问

http://www.5ykj.com/shti/cusan/25151.htm上面第四题是答案
但是W=xy=x（-80x+720）是怎么变成-80（x-9/2）的平方+1620

Answer 1

W＝xy＝x（－80x+720）= -80x²+720x= -80(x²+9x)= -80(x²-9x)= -80(x²-9x)= -80[x²-9x+(9/2)²-(9/2)²]
= -80(x-9/2)²+(-80)[-(9/2)²]= -80(x-9/2)²+1620

Answer 2

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