Question

find the 21 term of the A.P 7,11,15..​

Answer 1

EXPLANATION.

Series = 7, 11, 15,,,,,

As we know that,

First term = a = 7.

Common difference = d = b - a = c - b.

Common difference = d = 11 - 7 = 15 - 11.

Common difference = d = 4.

As we know that,

General term of an A.P.

⇒ Tₙ = a + (n - 1)d.

⇒ T₂₁ = a + (21 - 1)d.

⇒ T₂₁ = a + 20d.

⇒ T₂₁ = 7 + 20(4).

⇒ T₂₁ = 7 + 80.

⇒ T₂₁ = 87.

                                                                                                                             

MORE INFORMATION.

Supposition of terms in A.P.

(1) = Three terms as : a - d, a, a + d.

(2) = Four terms as : a - 3d, a - d, a + d, a + 3d.

(3) = Five terms as : a - 2d, a - d, a, a + d, a + 2d.

Answer 2

Given

• Arithmetic progression = 7 , 11 , 15 ..

To find

• 21 term of the A.P

Solution

In the given series,

• a = 7
• d = 15 - 11
• d = 4

Where,

• a is the first term.
• d is the difference between two consecutive terms.

We know, general term of an A.P :-

• Tₙ = a + (n - 1)d

Substituting we get :-

• Tₙ = 7 + (21 - 1)d
• T₂₁ = 7 + (20)d
• T₂₁ = 7 + 80
• T₂₁ = 87

Hence, 21 term of the A.P 7 , 11 , 15 is 87.