The sum of the first 10 terms of an AP is 30 and the sum of the first 40 terms is 90. Find the first term of the AP and the least value of r such that the rth term is negative. I need the complete and clear solution please.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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The sum of the first 10 terms of an AP is 30 and the sum of the first 40 terms is 90. Find the first term of the AP and the least value of r such that the rth term is negative.

I need the complete and clear solution please. 

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