Question

÷ a1, a2, 13, 17, 21. Determinați a1.

Answer 1

[tex] \div $ $ a_1, a_2,13,17,21.\\ \\ r = 17-13 = 4 \\ \\ \left\{ \begin{array}{ll} a_2 = \dfrac{13+a_1}{2} \\ a_2 = a_1+r = a_1+4= a_1 +4 \end{array} \right| \Rightarrow a_1+4 = \dfrac{13+a_1}{2} \Rightarrow \\ \\ \Rightarrow 2a_1+8 = 13+a_1 \Rightarrow 2a_1-a_1 = 13-8 \Rightarrow \boxed{\boxed{a_1 = 5}}[/tex]