a2-a1=2×2-8a3-a2=2×3-8a4-a3=2×4-8a5-a4=2×5-8················an-a(n-1)=2×n-8以上式子相加得an-a1=2(2+3+4+5+······+n)-8×(n-1)=2×(2+n)×(n-1)/2-8n+8=n²+n-2-8n+8=n²-7n+6所以an=a1+n²-7n+6把你漏写的a1代入即可。
