[คusักคณิm]

5.$1/(1*2)+1/(2*3)+1/(3*4)..........1/(99*100)$=เท่าไร

hint

$1/1(1/1-1/100)$

6.$1^2+2^2+3^2..........100^2$ หลักหน่วยคือ........

hint

$1^2+2^2...........n^2=1/6*n*(n+1)*(2n+1)$