Question

The second term of an arithmetic sequence is 13 and the fourth term is 7.
What is the thirteenth term?

Answer 1

Answer:

                $\bold{a_{13}=52}$

Step-by-step explanation:

$d=a_4-a_3=a_3-a_2\\\\a_3-a_2=a_4-a_3\\\\2a_3=a_2+a_4\\\\2a_3=13+7\\\\2a_3=20\\\\a_3=10\\\\d=10-13=-3\\\\\\a_2=a_1+d\quad\implies\quad a_1=a_2-d=13-(-3)=16\\\\a_n=a_1+d(n-1)\\\\a_{13}=16+(-3)(13-1)=16+36=52$