Tuesday, May 14, 2019

How to find the sum of the first 21 terms of an Arithmetic Progression(A.P)?


Question. If the sum of first 12 terms of an A.P. is equal to to the sum of the first 18 terms of the same A.P., find the sum of the first 21 terms of the same A.P.




a= first term



d= difference




I know now, 2(2a+11d)=3(2a+17d)



hence, 2a+29d=0 --> correction



How do I proceed from here?

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