[tex]\displaystyle\bf\\\textbf{Intr-o progresie aritmetica avem:}\\a_1=2\\a_{29}=-54\\Se~cere:\\r=~?\\a_{12}=~?\\\\Rezolvare:\\\\r=\frac{a_{29}-a_1}{29-1}=\frac{-54-2}{28}=\frac{-56}{28}=\boxed{\bf-2}\\\\a_{12}=a_1+(12-1)\cdot r=2+11\cdot(-2)=2-22=\boxed{\bf-20}[/tex]
