令a1=3,a2=7,a3=13,...规律：a2-a1=4=2*2, a3-a2=2*3, an- a(n-1)=2n;利用迭加可求出an .
则a99=9901
另设所求为x,则,100的三次方=100x+50(100-1),则x=9901