ARITHMETIC
PROGRESSIONS
4) Which term of the AP 3, 8, 13, 18, . . . is 78 ?
Sol:
We need to find which term no. is 78
For given AP: 3, 8, 13, 18, …
a = 3,
d =
8 – 3
= 5,
an = 78,
We know that,
an =
a + (n – 1) d
∴ 78 =
3
+ (n – 1)
( 5 )
∴ 78 – 3 =
(n – 1) 5
∴ 75 =
(n – 1) 5
∴ 15 =
n – 1
∴ n =
16
∴ 78 is the 16th term of AP.
i.e. find n when an = 78
Lets find value of a & d
Exercise 5.2 4
HOMEWORK