find the sum of 9+11+.....+79 what is Sn?

Question

anonymous · Answer

is it the sum of series of odd nos????

anonymous · Answer

I'm not sure, the problem only asks for the sum. I know that my a1=9 and difference ratio is probably 11/9 but I don't know where to go from there..

anonymous · Answer

are u sure diff ratio is 11/9 ???

anonymous · Answer

or its diff 11 - 9 ?

anonymous · Answer

any wayz....if its common diff is 2 then by formulae...
An= A +(N-1) D
we can find out the total no of terms
then we will have another formulae for sum 
Sn= N/2(2A+(N-1)D)

anonymous · Answer

oops! i was thinking of another formula! yes d=2. sorry, ok, now i know my answer is 1584 but i just need to make sure using the formula you provided. Thank you by the way :)

anonymous · Answer

got it!!!!! thank you so much! :)

anonymous · Answer

ur welcm dear....:)