Question

Find the 21st term in the ap whose first two terms are -3 and 4

Answer 1

$\huge\mathfrak{Solution}$

• Given that,

a = -3

d = 4 - (-3)

d = 7

n = 21

an = ?

• We know that,

an = a + (n-1)d

an = -3 + (21-1)7

an = -3 + 140

an = 137

Thus, the 21st term of the A. P is 137.

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Answer 2

Given,

AP is => -3, 4,11, 18.......

Here, a= - 3(first term )

D= 7(common difference)

n=21(given term)

Now, An= a +(n-1)d

=>A21 = - 3 + 20×(7)

=>A21 = - 3 + 140

=>A21 = 137

Hence the 21st term of the AP will be 137.

Remember

1)To find nth term of an AP, we have =>

An= a+(n-1)d

Where An= nth term

n=given term

d=common difference

a=first term of AP.

2)To find the sum of nth term of An AP we have =>

Sn = n/2{2a+(n-1)d}

Where Sn= sum till nth term

n=given term

d=common difference

a=first term of AP.