1. Un produs este egal cu 0 daca, unul din factori este 0 !!!
=>a) pt. x-3=0 obtinem x = 3∈ Q si pt. x+2=0 avem: x = -2∈Q ;
b) -4x+21x-5x = -3-2 ⇔ x = -5 / +12 = - 5/12 ∈ Q ;
2. Vom folosii invers formula:
1/n - 1/ n+1 = (n+1-n) / n(n+1) = n / n*(n+1) ;
[tex]1/1\underbrace{-1/2+1/2-1/3+1/3-1/4+...+1/x}_{=0}-\frac{1}{x+1}=2010/201\\
\Rightarrow\;\;\frac{1}{1}-\frac{1}{x+1}=\frac{2010}{2011}\\
\frac{x+1-1}{x+1}=\frac{2010}{2011}\;\;\Rightarrow\;\;\fbox{x=2010} [/tex]
