यदि `(1+x)^(n)=underset(r=0)overset(n)sum C_(r).x^(r )" तो सिद्ध कीजिये : "(2^(2).C_(0))/(1.2)+(2^(3).C_(1))/(2.3)+(2^(4).C_(2))/(3.4)+…….+(2^(n+2).C_(n))/((n+1)(n+2))=(3^(n+2)-2n-5)/((n+1)(n+2))`