Question -
Answer -
A.P. is given : 3, 15, 27, 39, …….
Here first term (a) = 3
and c.d. (d) = 15 – 3 = 12
Let nth term be the required term
Now 21st term = a + (n – 1) d = 3 + 20 x 12 = 3 + 240 = 243
According to the given condition,
nth term – 21 st term = 120
=> a + (n – 1) d – 243 = 120
=> 3 + (n – 1) x 12 = 120 + 243 = 363
=> (n – 1) 12 = 363 – 3 = 360
=> n – 1 = 30
=> n = 30 + 1 = 31
31 st term is the required term