✔ 最佳答案
1.(A)25*13^2-15^2=4000
(B)16*17^2-18^2=4300
(C)9*21^2-13^2=3800
(D)4*31^2-12^2=3700
所以(B)最大
2.2^x=8^(y+1)
2^x=(2^3)^(y+1)
2^x=2^3(y+1)
x=3(y+1)
x=3y+3
x-3y=3--------------------(1)
9^y=3^(x-9)
(3^2)^y=3^(x-9)
3^2y=3^(x-9)
2y=x-9
x-2y=9-----------------(2)
(2)-(1) (x-2y)-(x-3y)=9-3
x-2y-x+3y=6
y=6
代y=6入(1)
x-3(6)=3
x=21
(x+y)(x-y)
=(21+6)(21-6)
=405