3^(n+2) - 2^(n+3) + 3^n - 2^(n+1)
=(3^2*3^n+3^n)-(2^3*2^n+2*2^n)
=3^n(3^2+1)-2^n(2^3+2)
=10*3^n-10*2^n
=10*(3^n-2^n)
所以一定能被10整除
a-b=2,a-c=0.5
相减
(a-b)-(a-c)=2-0.5
c-b=1.5
b-c=-1.5
（b-c）^2 - 3(b-c)+9／4
=2.25+4.5+2.25
=9
(3x-y-2)(2x-y-1).
=6x^2-3xy-3x-2xy+y^2+y-4x+2y+2
=6x^2-5xy+y^2-7x+3y+2
=6x^2-5xy+y^2+mx+ny+2
对应项系数相等
所以m=-7,n=3