1/3+1/6+1/10+....+2/x×(x+1)=2005/2007
⇔26 +212 +220 +...+2x(x+1) =2005/2007
⇔22.3 +23.4 +24.5 +...+2x(x+1) =2005/2006
⇔2.(12.3 +13.4 +14.5 +...+1x(x+1) )=2005/2007
⇒2.(12 −13 +13 −14 +14 −15 +...+1x −1x+1 )=2005/2007
⇒2.(12 −1x+1 )=2005/2007
⇒12 −1x+1 =2005/2007 :2
⇒12 −1x+1 =2005/4014
⇒1x+1 =12 −2005/4014
⇒1x+1 =1/2007
⇒x+1=1/2007
=> x = 2007 - 1
=> x = 2006
chúc bn học tốt!!!
