Question

a1, a2 ,19 ,25,.... a2, a1=?

Answer 1

$a_{3} = 19$

$a_{4} = 25$

$r = a_{4} - a_{3} = 25 - 19 = 6$

$a_{2} = a_{3} - r = 19 - 6 = 13$

$a_{1} = a_{2} - r = 13 - 6 = 7$