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In an ap the sum of …

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In an ap the sum of first ten terms is -80 and the sum of its next ten terms is (-280 ) find the ap

Posted by Rihan Ansari 7 years, 7 months ago

CBSE > Class 10 > Mathematics

1 answers

V Sridhar 7 years, 7 months ago

The series will be 1, -1, -3, -5, -7,........

How to get it ?

The sum of n terms of an AP is given by , S = n/2 {2a+(n-1)d}

According to problem , S (10) = sum of first ten terms = -80 = 10/2 {2a + (10-1)d} = 5 (2a + 9d)------------(1)
Again sum of next 10 terms is = -280
Therefore we can write , S (20) = sum of first twenty terms = -80-280 = 20/2{20+(19-1)d} = 10(2a+19d)---------(2)

solving (1) and (2) we get a= first term = 1 and d= common difference = -2.

2Thank You

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CBSE > Class 10 > Mathematics