Question

If the 3rd and the 5th term of an arithmetic progression are 13 and 21, what is the 13th term?

A) 53 B) 49 C) 57 D) 61

Answer 1

A3 = 13

a + 2d = 13

A5 = 21

a + 4d = 21

a + 2d = 13 – ( 1 )

a + 4d = 21 – ( 2 )

__________

– 2d = – 8

d = 8/2

d = 4

put d = 4 in ( 1 )

a + 8 = 13

a = 13 – 8

a = 5

A13 = a + 12d

= 5 + 12 (4)

= 5 + 48

= 53

( A ) is the correct answer.