p+2的绝对值与q平方-8q+16互为相反数
即
p+2的绝对值+q平方-8q+16=0
所以
p+2=0,(q-4)²=0
p=-2,q=4
x^2*y^2-(2pxy-q)
=x²y²-(-4xy-4)
=x²y²+4xy+4
=(xy+2)²