Question

Find the sum of all two digit numbers

Answer 1

there u are... 4905

Answer 2

Here is your solution:-

The two digit numbers are :-

10,11,12,...........99

Now it is clear that the given sequence is in AP with difference of 1

So, a=10 ; d=1 & an= L = 99

Therefore,

an = a + (n-1)×d

99= 10 + (n-1)×1

99-10=n-1

89=n-1

n=89+1

n=90

So there are total 90 two digit numbers

Now,

Sn=n/2(a+L)

S20= 20/2(10+99)

S20=10(109)

S20 = 1090

Therefore the sum of all two digit numbers is 1090.---(Answer)

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