[tex]\displaystyle\\
\text{Se da progresia aritmetica: } \\
x,~y,~-21,~z,~-15,~t~,..... \\ \\
Se cere:\\
x = ?,~y=?,~z=?,~t=?\\\\
\text{Rezolvare: }\\\\
z = \frac{-21 + (-15)}{2} = \frac{-21 -15}{2} = \frac{-36}{2} = \boxed{-18} \\ \\
\Longrightarrow~~r = -15-z = -15-(-18) = -15 +18 = \boxed{3 ~~(ratia)}\\
t = -15 + r = -15 + 3 = \boxed{-12}\\
y = -21-r = -21 -3 = \boxed{-24}\\
x = y - r = -24 -3 = \boxed{-27}
[/tex]