Question 1054496
{{{S= (1/2)1(1+1)+(1/2)2(2+1)+(1/2)3(3+1)+(1/2)4(4+1)}}}
{{{S= (1/2)(1*2+2*3+3*4+4*5)}}}
{{{S= (1/2)(2+6+12+20)}}}
{{{S=1+3+6+10}}}
{{{S=20}}}