已知x+y=5,xy=2,求x的八次方+y的八次方的值,

x的八次方+y的八次方
=（x的4 次方+y的4次方）²-2x^4y^4
=【（x²+y²）²-2x²y²】²-2x^4y^4
={【（x+y）²-2xy】²-2x²y²}²-2x²y²}-2x^4y^4
={【5²-2×2】²-2×2²}²-2×2^4
={21²-8}²-32
=1874572x^4y^4是乘方？初一的题目，用乘法公式解2x的4次方y的4次方我还以为2x的4y次方。。。。这样是简写