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In an A.P , the sum of first ten terms is (-80) and the sum of its next ten terms is (-280)Find the A.P

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So plug in your formula. If S1 is the sum of the 1st ten terms, and S2 is the sum of the 1st twenty terms, then you have
S1 = 10/2 (2a+9d) = -80
S2 = 20/2 (2a+19d) - S1 = -280
Rearranged a bit, that's
10a + 45d = -80
10a + 145d = -280
Looks like
a = 1
d = -2
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