If the sum of the first k terms of the arithmetic sequence {an} is 30 and the sum of the first 2K terms is 100, then the sum of the first 3K terms of the arithmetic sequence {an} is?
SK, S2K SK, s3k-s2k of arithmetic sequence {an} are still arithmetic sequence
So 30100-30 and s3k-100 are arithmetic sequences
SO 2 * 70 = 30 + s3k-100
S3k=210
So the sum of the first 3K terms is 210
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