Pentagonal Numbers Formula

Hi everyone

I have been asked to find General term for Pentagonal numbers in one of our Exams. I have solved the general term for the pentagonal numbers by subtracting the same series from itself. The procedure is as follows

S = 1 + 5 + 12 + 22 + 35 + .......................................+ tn

S = 0 + 1 + 5 + 12 + 22 + 35 + .......................................+ tn

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0 = 1 + 4 + 7 + 10 + 13 + ............................................. - tn

=> tn = 1 + 4 + 7 + 10 + 13 + ............................................. [ Sum of n terms of A.P with common difference 3 and first term 1]

tn = (n/2) (3n - 1)

I hope the method is absolutely correct to find the general term....

Am I correct in doing so???

If I am correct Then ----

**I have got 0 [zero] marks out of 6 marks for this question **

My Instructor is not convinced with the method that i have adopted i.e subtracting a series from the same series itself ....

He says it can be implemented only for finite series..

Please give me a valid Link which uses the same procedure that i have adopted..

OR

Any link That supports my method ...

So that i can show the link to my instructor to get my marks...

Is there any specific name to the method that i have adopted ???

Examples which use this method are required urgently...???

I kindly request everyone to Give links that support my method

THANKS IN ADVANCE

bye

Please give me some link that support my method

HI **Soroban**

Thanks for your quick reply....

Sorry for the mistake that

**He says it can be implemented only for finite series.**

He says it can be implemented only for infinite series

Please give me a valid Link which uses the same procedure that i have adopted..

OR

Any link That supports my method ...

I am badly in need of those 6 marks ...

I may miss my **A** grade because of that.........

Bye