Question

the sum of all natural Numbers between 200 and 400 which are divisible by 7 is

Answer 1

Hey!!!

Required AP : 203, 210, 217 .. 399

Thus
a = 203
d = 7
l = 399
n = ?

l = a + (n - 1)d

=> 399 = 203 + (n - 1)7

=> (n - 1)7 = 196

=> n - 1 = 28

=> n = 29

S29 = n/2(a + l)

=> 29/2( 203 + 399 )

=> 29/2(602)

=> 29 x 301

=> 8729

Hope this helps

Answer 2

203,210,217..........399
an=a+(n-1)d
399=203+(n-1)7
399-203=(n-1)7
196/7=n-1
28=n-1
n=29

Sn=n/2(a+an)
S=29/2(203+399)
S=29/2(602)
S=29(301)
S=8729