The sum of first n terms of AP is 3n² + 6n . Find n th term and 15th term.
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in order to find n'th term in an AP we need to subtract the sum of "n" terms with sum of "n-1" terms. This gives us the "n'th" term of that AP. Therefore in this question we replaced "n" by "n-1" to get the sum of "n-1" terms and then proceeded as shown in the picture.
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