Question

cho A =1/2+1/6+1/12+1/20+1/30+.............+1/n,biết A =39/40 .tìm N

 

Answer 1

đặt n=k.(k-1)

ta có A=1/2+1/6+1/12+..............+1/n

A=1/1.2+1/2.3+1/3.4+................+1/(k-1).k

A=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+..................+1/k-1+1/k

A=1-1/k

A=k-1/k=39/40

suy ra k=40 : k-1=39

suy ra n=39.40=1560