1、已知x+4y=-1,xy=5,求(6xy+7y)+[8x-(5xy-y+6x)]的值.
6xy+7y+8x-5xy+y-6x
=xy+2x+8y
=5+2(x+4y)
=5+2*(-1)
=3
2、已知x的3次方=m,x的5次方=n,用m,n表示x的22次方.
x^22=(x^12)*x^10=(x^3)^4*(x^5)^2=m^4n^2
3、已知n为正整数,且x的2n次方=7,求(2x的3n次方)的2次方-3(x的2次方)n的次方.
(2x^3n)^2-3(x^2)^n
=4x^6n-3x^2n
=4(x^2n)^3-3*7
=4*7^3-21
=1351