(x^m÷x^2×n)^3÷x^（m-n）=(x^(m-2n))^3÷x^（m-n）=x^3(m-2n)÷x^（m-n）=x^(3m-6n-m+n)=x^(2m-5n)与-1/4x^2为同类项所以2m-5n=2因为2m+5n=7所以4m=9m=9/4n=1/24m²-2n²=4*(81/16)-2*(1/4)=81/4 -1/2=79...