The sum of 10 terms of the arithmetic series is 390. If the third term of the series is 19. Find the first term :
A. 3
B. 5
C. 7
D. 8
Answer: Option A
Solution(By Examveda Team)
Sum of A.P. = $$\frac{n}{2}\left[ {2a + (n - 1)d} \right]$$nth term = a + (n - 1)d
3rd term :
⇒ a + (3 - 1)d = 19
⇒ a + 2d = 19.....(i)
Sum of 10 term :
$$\eqalign{ & \Rightarrow \frac{{10}}{2}\left[ {2a + 9d} \right] = 390 \cr & \Rightarrow 2a + 9d = 78..... (ii) \cr & \Rightarrow a + 2d = 19 \cr} $$
From equation (i) and (ii)
a = 3
d = 8
Related Questions on Number System
Three numbers are in ratio 1 : 2 : 3 and HCF is 12. The numbers are:
A. 12, 24, 36
B. 11, 22, 33
C. 12, 24, 32
D. 5, 10, 15
Join The Discussion