Question

A.P.3, 8, 13, 18, ... का कौन-सा पद 78 है?

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Answer 1

Answer:

यहां a=3,d=2Nd term -1st term

d=5

an=78

हमें n निकालना है ।

using formula

an =a+(n-1)d

78=3+(n-1)5

78=3+5n-5

78-3+5=5n

78+2=5n

80=5n

5n=80

n=16

इसलिए 78 संख्या है=A.P का 16 वां पद।

Answer 2

Answer:

16

Step-by-step explanation:

Given---> AP , 3 , 8 , 13 , 18

To find ---> Which term of AP is 78

Solution---> 3 , 8 , 13 , 18 , .....................

First term = 3

a = 3

Common difference = a₂ - a₁ = 8 - 3 = 5

Now we have a formula of nth term of AP as follows

aₙ = a + ( n -1 ) d

Let nth term of AP be 78 , then

Here , a = 3 , d = 5 , n = , aₙ= 78

=> 78 = 3 + ( n - 1 ) ×5

= 3 + ( n - 1 ) × 5

= 3 + 5n - 5

78 = 5 n - 2

5 n = 78 + 2

5 n = 80

n = 80 / 5

n = 16

Additional information --->

(1) Formula of sum of n terms of AP

Sₙ = n / 2 { 2a + ( n - 1 ) d }

(2 ) Formula of sum of n terms of AP if last term is given

Sₙ = n / 2 ( a + l )

(3) Formula of nth terms of an AP from last

aₙ¹ = l - ( n - 1 ) d