Question

find the sum of all two digit odd positive no.s

Answer 1

  A.P. formed=11,13,15,17...........99

 a = 11 

d = 2

tn= 99

n = ?

Sum of  n terms = (n/2)[2a+(n -1)d]

 tn = a + (n -1)d

 99 = 11+ (n-1)2

99 -11 = (n-1)2

 88/2 = (n-1)

 n = 45.

subsitute n = 45 

 s45 = (45/2)(2×11 + (45 -1)2)

s45 = (45/2)(110)

 s45 = 45×55.

s45 = 2475.

So the answer is 2475.