Question

Проверьте, верно ли равенство: а) 1/ 6•7+ 1/ 7•8+1/ 8•9=(1/6-1/6)+1/7-1/8)+(1/8-1/9) б) 1/ 5•9+1/ 9•13+ 1/ 13•17=(1/5-1/9)+(1/9-1/13)+(1/13-1/17) в) 2/3+2/15+2/35=(1-1/3)+(1/3-1/5)+(1/5-1/7)

Answer 1

Ответ:

1/(5*6)+1/(6*7)+1/(7*8)+...1/(18*19)+1/(19*20)=

1/5-1/6+1/6-1/7+1/7-1/8+...1/18-1/19+1/19-1/20=

1/5-1/20=3/20

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