当n为偶数时,
c[n-1]=3^(n-1)-m*2^(n-1)
c[n]=3^n+m*2^n
c[n+1]=3^(n+1)-m*2^(n+1)
因为c[n+1]>c[n]>c[n-1]
所以-2*3^(n-1)-(3/2)^(n-2)
3m*2^n
当n为偶数时,
c[n-1]=3^(n-1)-m*2^(n-1)
c[n]=3^n+m*2^n
c[n+1]=3^(n+1)-m*2^(n+1)
因为c[n+1]>c[n]>c[n-1]
所以-2*3^(n-1)-(3/2)^(n-2)
3m*2^n