Question

giup to to like

1/2+1/6+1/12+1/20+...+1/90

Answer 1

1/2 + 1/6 + 1/12 + 1/20 +...1/90 = 1/1.2 + 1/2.3+ 1/3.4 +...+1/9.10

= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - ...-1/10

= 1/1 - 1/10 = 9/10

Answer 2

Giải pt 
(1+1/2)+(1+1/6)+(1+1/12)+(1+1/20)+. 
..+{1+1/[x(x+1)]}=8,9 
<=>x+1/2+1/6+...+1/[x(x+1)]=8,9 
<=>x+1/(1*2)+1/(2*3)+...+1/[x(x+1)]=8,... 
<=>x+1-1/2+1/2-1/3+..+1/x-1/(x+1)=8,9 
<=>x+x/(x+1)=8,9 
<=>x^2+2x=8,9x+8,9<=>x=8