依题意得:(b-3a)与(3a-b)互为相反数
(b-3a)²=(3a-b)²
∴原式=【(3a-b)²】^(n+1)+(3a-b)^(6n+3)
=(3a-b)^(2n+2)+(3a-b)^(6n+3)
=(3a-b)^(8n+5)