因为：a1,a3,a6成等比数列所以：a3^2 =a1 × a6(a1+2d)^2=a1×(a1+5d)a1^2 + 4a1 × d + 4 × d^2 = a1^2 + 5 × a1 × d得到：a1×d = 4×d×d又,d不为0所以：d = a1/4 = 1/2所以Sn = a1+(n-1)d/2=(n^2 + 7n)/4...