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2014-06-20T12:38:44+05:30

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Let the three terms in A.P be a - d,a, a + d
where d is the common difference
given that sum of the three terms is 21 so     a - d + a + a + d= 21
                                                                  ⇒3a = 21 ⇒ a = 7
now given (a - d)(a + d) = a + 6
                ⇒ a² - d² = a + 6
                ⇒ 7² - d ² = 13
                ⇒d =  + 6, - 6
so third term(a + d) if  +6 is taken is 13 and if  - 6 is taken is 1 
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