a的2n+2次方乘b的2n次方乘b的4次方
=a的2n+2次方·b的2n+2次方
=5的2n+2次方·(-5分之1)的2n+2次方
=5的2n+2次方·(5分之1)的2n+2次方
=5的2n+2次方·5的-2n-2次方
=（5）的0次方
=1