Question

A series is given by 51+50+49+...........+21.how many terms are there in the series??

Answer 1

This could be the required answer. ☝️

Answer 2

Answer:

1116

Step-by-step explanation:

We can invert the same to obtain the results :

Hence , series will be :

21 + 22 + 23 + ... + 51

a = 21

d = 22 - 21 = 1

T( n ) = a + (n - 1)d = 51

21 + n - 1 = 51

n + 20 = 51

n = 51 - 20 = 31

Now , summation of the series :

S(n) = n/2 [ 2a + ( n - 1 )d ]

31 /2 [ 2 ( 21 ) + (31 - 1) ]

31/2 [ 42 + 30 ] = 31/2 [ 72 ]

= 1116