Question

Find the sum of all two digit odd multiples of 3?​

Answer 1

Two digit odd multiples of 3 is 15,18,21,....,99.

Here a = 15 , l = 99 , n = 15.

Sn = n/2(a+l)

S15 = 15/2(15+99)

= 15/2(114)

=855.

Answer 2

Answer:

the sum of all two digit odd multiples of 3 = 855

Explanation:

15,21,....,99 are two digit odd

multiples of 3 are in A.P.

first term (a) = 15

Common difference (d) =a2-a1

= 21 - 15

= 6

i ) nth term (an) = 99

=> a+(n-1)d = 99

=> 15 + (n-1)6 = 99

=> (n-1)6 = 99-15

=> (n-1)6 = 84

=> n-1= 84/6

=> n-1 = 14

=> n = 15

ii ) sum of n terms (Sn) =

(n/2)[a+an]

S15 =(15/2)[15+99]

= (15/2)[114]

= 15 × 57

= 855

Therefore,

the sum of all two digit odd multiples of 3 = 855

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