[tex]a_1 = x-6\\a_2 = x\\a_3 = 13[/tex]
Z własności ciągu arytmetycznego:
[tex]a_2 = \frac{a_1+a_3}{2}\\\\x = \frac{x-6+13}{2}\\\\x = \frac{x+7}{2} \ \ /\cdot2\\\\2x=x+7\\\\2x-x = 7\\\\x = 7\\\\\\a_1 = 7-6 = 1\\\\a_2 = 7\\\\a_3 = 13\\\\\\r = a_3 - a_2 = a_2 - a_1\\\\r = 13-7 = 7-1\\\\r = 6[/tex]
