1／2+1／6+1／12+...+1／k²+k
=1/1x2+1/2x3+1/3x4+……+1/k(k+1)
=1-1/2+1/2-1/3+1/3-1/4+……+1/k-1/(k+1)
=1-1/(k+1)
=k/(k+1)