Find the sum of all two digit positive integers which are divisible by 3 but not by 2
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The AP is 12,15, 18....,99
first term a=12
Common difference d=3
an =99
a+(n-1)d= an
12+(n-1)3=99
(n-1)3=87
(n-1)=29
n=30
sum of an AP=n/2(a+l). (When last term is given this formula can be used)
=30/2(12+99)
=15*111
=1665
The sum of the AP 12, 15, 18,......99 is 1665
first term a=12
Common difference d=3
an =99
a+(n-1)d= an
12+(n-1)3=99
(n-1)3=87
(n-1)=29
n=30
sum of an AP=n/2(a+l). (When last term is given this formula can be used)
=30/2(12+99)
=15*111
=1665
The sum of the AP 12, 15, 18,......99 is 1665
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