Given series is 1+21+2+31+3+41+…..nth term =n+n+11∴15th term =15+161 Thus, given series upto 15 terms is 1+21+2+31+3+41+……+15+161 This can be re-written as −11−2+−12−3+−13−4+……+−115−16 (By rationalization) =−1+2−2+3−3+4+…..−14+15−15+16=−1+16=−1+4=3 Hence, the required sum =3