Odpowiedź:
[tex]a_{3}=-4\ \ \ \ a_{7}=8\\\\\\r=\dfrac{a_{7}-a_{3}}{7-3}=\dfrac{8-(-4)}{4}=\dfrac{8+4}{4}=\dfrac{12}{4}=3\\\\\\a_{n}=a_{1}+(n-1)*r\\\\a_{3}=a_{1}+(3-1)*3\\\\-4=a_{1}+2*3\\\\-4=a_{1}+6\\\\-4-6=a_{1}\\\\-10=a_{1}\\\\a_{1}=-10\\\\\\Odp.Pierwszy\ \ wyraz\ \ tego\ \ ciagu\ \ wynosi\ \ a_{1}=-10,\ \ a\ \ r\'o\.znica\ \ r=3[/tex]