Question

3.18 пожалуйста помогите.

Answer 1

An=A₁+d(n-1)
$left { {{ a_{3} = a_{1}+2d } atop { a_{12}= a_{1}+11d }} right. \ left { {{ -2 = a_{1}+2d |*(-1) } atop { -38= a_{1}+11d }} right. \ left { {{ 2 = -a_{1}-2d } atop { -38= a_{1}+11d }} right. \ -36=9d \ d=-36/9=-4$
отв:-4