Question

find the sum of all two digit positive numbers divisible by 3

Answer 1

2 digit no. divisble by 3 are     12,15,18,21..............99

so ,  a=12  d=3  an=99

an = a + (n-1)d

99 = 12 + (n-1) 3

99-12 = (n-1) 3

87/3 = (n-1)

29 +1 =n 

  n= 30

Sn = n/2 (a+l)

  = 30/2 (12 + 99)

  = 15 * 111 

  =  1665