设a[n] = 7^n-4^n-3^n.n = 1时,a[1] = 0被12整除,结论成立.假设n = k时a[k]被12整除.a[k+1] = 7^(k+1)-4^(k+1)-3^(k+1)= 7·7^k-4·4^k-3·3^k= 7(a[k]+4^k+3^k)-4·4^k-3·3^k= 7a[k]+3·4^k+4·3^k= 7a[k]+12·(4^...