Q. 315.0( 1 Vote )

Find the sum of first 21 terms of the A.P. whose 2nd term is 8 and 4th term is 14.

Answer :

Given: a2 = 8 and a4 = 14 and n = 21


We know that,


a2 = a + d = 8 …(i)


and a4 = a + 3d = 14 …(ii)


Solving the linear equations (i) and (ii), we get


a + d – a – 3d = 8 – 14


-2d = -6


d = 3


Putting the value of d in eq. (i), we get


a + 3 = 8


a = 8 – 3 = 5


Now, we have to find the sum of first 21 terms.





S21 = 21 [5 + 10 × (3)]


S21 = 21 [35]


S21 = 735


Rate this question :

How useful is this solution?
We strive to provide quality solutions. Please rate us to serve you better.
Related Videos
Champ Quiz | Arithmetic Progression34 mins
Champ Quiz | Arithmetic Progression30 mins
Lets Check Your Knowledge in A.P.49 mins
Fundamental Theorem of Arithmetic- 143 mins
NCERT | Fundamental Theorem Of Arithmetic45 mins
Quiz on Arithmetic Progression Quiz32 mins
Arithmetic progression: Previous Year NTSE Questions34 mins
Get to Know About Geometric Progression41 mins
NCERT | Solving Questions on Introduction of A.P42 mins
Arithmetic Progression Tricks and QUIZ37 mins
Try our Mini CourseMaster Important Topics in 7 DaysLearn from IITians, NITians, Doctors & Academic Experts
Dedicated counsellor for each student
24X7 Doubt Resolution
Daily Report Card
Detailed Performance Evaluation
view all courses