Given, an+1=an+4n and a1=4     ⇒‌‌an+1−an=4n Put n=1   a2−a1=4.1 Put n=2   a3−a2=4.2 Put n=3   a4−a3=4.3 ............    ......  ..... ............    ......  ..... Put n=99  a100−a99=4.99 On adding, we get a100−a1=4(1+2+...+99)