Question

find the sum of all two digit odd positive number

Answer 1

smallest 2 digit's odd positive number=11,

greatest 2 digit's odd positive number=99,

so the series will be 11,13,15,..........97,99,

which form an A.P,

then
number of terms=(99-11)/2 +1,

=88/2 +1,

=44+1=45,
then
sum of all two digit's positive odd numbers=(11+99)×45/2,

=110 ×45/2,

=55×45=2475

Answer 2

Hey dear friend

Given A.P.= 11,13,15,.,..,.,.99.
a=11, d=2 and An(l)=99

An(l)=a+(n-1)d
99=11+(n-1)2
99-11=(n-1)2
88/2=n-1
44=n-1
n=45

So, By the formula of sum
Sn=n/2(a+l)
Sn=45/2(11+99)
Sn=45/2(110)
Sn=45*55
Sn=2475

So, the sum of all two digit pdd positive number is 2475.

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