Question

Which term of the AP 21,42,63,84,...... is 120?

Answer 1

Step-by-step explanation:

given that ,

first term = 21

common difference = 42 - 21 = 21

last term = 120

then ,

an = a + ( n - 1 ) d

120 = 21 + ( n - 1 ) 21

120 - 21 = 21n - 21

99 = 21n - 21

99 + 21 = 21n

120 = 21n

n = 120 / 21

n = 40 / 7