x²-8x+a²-12a+51=0
x²-8x+16+a²-12a+36=1
(x-4)²+(a-6)²=1
平方项恒非负,0≤(x-4)²≤1 0≤(a-6)²≤1
x-4=0时,即x=4时,(a-6)²=1 a-6=1或-1 a=7或a=5
(x-4)²=1时,即x=5或x=3时,(a-6)²=0 a-6=0 a=6