Natural numbers are divided into groups $\;(1)\;,\;(2,3,4)\;,\;(5,6,7,8,9)\;.....$ Sum of first and last term of $n^{th}\;$ group will be :

Question

$(a)\;n^2+n\qquad(b)\;n^2-n+2\qquad(c)\;2n^2-2n+2\qquad(d)\;n^2-2n+2$