Question

5. Find the sum of the integers between 100 and 200 that are

(i) divisible by 9

(ii) not divisible by 9

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Answer 1

(i) a=108 d=9 an=l=198

an=a+(n-1)*d

198=108+(n-1)*9

90=(n-1)*9

10=n-1

n=11

(ii)Sn=n/2(2a+(n-1)rd)

       =11/2(2*108+(11-1)*9)

       =11/2(216+10*9)

       =11/2(216+90)

       =11/2(306)

       =1683