1.

Find the sum of all integers between 84 and 719, which are multiples of 5

Answer»

The series is 85, 90, 95, …, 715

Let there be ‘n’ terms in the AP

So, a = 85, d = 90-85 = 5, an = 715

an = a + (n - 1)d

715 = 85 + (n - 1)5

715 = 85 + 5n – 5

5n = 715 – 85 + 5

5n = 635

n = 635/5

= 127

By using the formula,

Sum of n terms, S = n/2 [a + l]

= 127/2 [85 + 715]

= 127/2 [800]

= 127 [400]

= 50800

∴ The sum of all integers between 84 and 719, which are multiples of 5 is 50800.



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