1
A=$\frac{7}{10}$+ ($\frac{7}{10})^{2}$+...+ ($\frac{7}{10})^{n}$
$\frac{7}{10}$ A= ($\frac{7}{10})^{2}$ ($\frac{7}{10})^{3}$+...+ ($\frac{7}{10})^{n+1}$
⇒ $\frac{7}{10}$A -A = ($\frac{7}{10})^{2}$ ($\frac{7}{10})^{3}$+...+ ($\frac{7}{10})^{n+1}$ -$\frac{7}{10}$+ ($\frac{7}{10})^{2}$+...+ ($\frac{7}{10})^{n}$
$\frac{-3}{10}$A = $\frac{7}{10}$-($\frac{7}{10})^{n+1}$ ⇒ A =$\frac{ \frac{7}{10}-(\frac{7}{10})^{n+1} }{-3}$
2
A=$\frac{-5x}{21}$+$\frac{-5y}{21}$+$\frac{-5z}{21}$
A= $\frac{-5x-5y-5z}{21}$ =$\frac{-5*(x+y)-5z}{21}$= $\frac{-5*(-z)-5z}{y}$ =$\frac{5z-5z}{21}$=0
