2C(n,5)=C(n,4)+C(n,6)2n!/[5!(n-5)!]=n!/[4!(n-4)!]+n!/[6!(n-6)!]两边约去n!,同时乘以6!(n-4)!12(n-4)=30+(n-5)(n-4)n^2-21n+90=0n=16,n=5(舍去）∴n=16∴C(16,12)=C(16,4)=1820n^2-21n+90=0这步算错了，是n²-21n+98=0,是不是？最终答案是91