Subjects

Mathematics Science Social Science Civics Social Science Economics Social Science Geography Social Science History

Chapter 5 · Question 2

State the formula for the $n$ th term of an AP and use it to find the 20th term of $3,7,11,15,\ldots$ .

Back to Chapter

Q2

State the formula for the $n$ th term of an AP and use it to find the 20th term of $3,7,11,15,\ldots$ .

Answer Revealed

Direct Answer:

For an AP with first term

a

and common difference

d

,

a_n=a+(n-1)d

. Here

a=3

,

d=4

, so

a_{20}=3+19\times4=79

.

Simple Explanation

Use

a_n=a+(n-1)d

. The 20th term is

79

.

Exam-Ready Structure

The first term is

a=3

and common difference

d=7-3=4

. Thus

a_n=a+(n-1)d

. For

n=20

,

a_{20}=3+(20-1)4=3+76=79

.

Key Points

For an AP with first term $a$ and common difference $d$ , $a_n=a+(n-1)d$ .
Use the NCERT formula or theorem carefully.
Write units and final conclusion where applicable.

Related Questions

How many terms are there in the AP $7,10,13,\ldots,43$ ?

State the formula for the sum of first $n$ terms of an AP. Find the sum of first 20 natural numbers.

Report a mistake