a1+2a2+3a3+4a4+……+(n-1)a + nan=n(n+1)(n+2）, a1+2a2+3a3+4a4+……+(n-1)a = (n-1)n(n+1) 两式相减 n an = n(n+1)(n+2) - (n-1)n(n+1) = 3n(n+1) an = 3(n+1)
