Cho A= \(\dfrac{1}{5}+\dfrac{1}{14}+\dfrac{1}{30}+\dfrac{1}{50}+\dfrac{1}{63}+\dfrac{1}{70}+\dfrac{1}{98}\)

Chứng Minh A < \(\dfrac{1}{2}\)

Cho \(\dfrac{bz-cy}{a}\)=\(\dfrac{cx-az}{b}\)=\(\dfrac{ay-bx}{c}\). Chứng minh \(\dfrac{x}{a}\)=\(\dfrac{y}{b}\)=\(\dfrac{z}{c}\)