21×3+33×6+46×10+510×15+615×21．

Question

(2)/(1×3)+(3)/(3×6)+(4)/(6×10)+(5)/(10×15)+(6)/(15×21)．

Answer 1

(2)/(1×3)+(3)/(3×6)+(4)/(6×10)+(5)/(10×15)+(6)/(15×21)，
=(1)/(1)−(1)/(3)+(1)/(3)−(1)/(6)+(1)/(6)−(1)/(10)+(1)/(10)−(1)/(15)+(1)/(15)−(1)/(21)
=1-(1)/(21)，
=(20)/(21)．