Question

Find the sum of all multiples of 7 lying between 300 and 700

Answer 1

Hey!!!!!

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Let's form an AP first

=> 301,308,315,...693

Here

a = 301
d = 7
l = 693
n = ?

Thus We know

=> l = a + (n - 1)d

=> 693 = 301 + (n - 1)7

=> (n - 1)7 = 392

=> n - 1 = 58

=> n = 59

Now S60 = ?

=> S60 =
$\frac{n}{2} (a + l)$

$= > \frac{59}{2} (301 + 693)$
$= > \frac{59}{2}(994)$

Thus the required answer = 59 x 497
=> 29323

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Hope this helps ✌️