(x+2y)²-10(x+2y)+25
=(x+2y-5)²
9（2x-y）²-6(2x-y)+1
=(3(2x-y)-1)²
=(6x-3y-1)²
(x+a)²-6(x+a)(x-b)+9(x-b)²
=(x+a-3x+3b)²
=(a+3b-2x)²
（p+q）²-2r(p+q)+r²
=(p+q-r)²
这些题目考的都是(a-b)²=a²-2ab+b²