# Sum of n terms of series $\;S=1^2+2(2)^2+3^2+2(4)^2+5^2+\;....\;$ when n is even is :
$(a)\;\frac{1}{4}n(n+2)^2\qquad(b)\;\frac{1}{4}n^2(n+2)\qquad(c)\;\frac{1}{2}n^2(n+1)\qquad(d)\;\frac{1}{2}n(n+1)^2$