How many terms of the AP: –15, –13, –11,— are needed to make the sum –55? Explain the reason for double answer.
Find the sum of first seven numbers which are multiples of 2 as well as of 9.
If sum of first 6 terms of an AP is 36 and that of the first 16 terms is 256, find the sum of first 10 terms.
Find the sum of first 17 terms of an AP whose 4th and 9th terms are –15 and –30 respectively.
If Sn denotes the sum of first n terms of an AP, prove that S12 = 3(S8 –S4)
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