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If a, b, c are in A.P. and a2, b2, c2 are in H.P., then which of the following statement can be true?(a) a, b, – \(\frac{c}{2}\) are in G.P. (b) a = b = c (c) Any of these (d) None of these |
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Answer» (c) Any of these a, b, c are in A.P. ⇒ 2b = a + c ...(i) a2, b2, c2 are in H.P. ⇒ b2 = \(\frac{2a^2c^2}{a^2+c^2}\) ......(ii) From eqn (ii) b2 (a2 + c2) = 2a2c2 ⇒ b2 {(a + c)2 – 2ac} = 2a2c2 ⇒ b2 {4b2 – 2ac} = 2a2c2 (From (i) 2b = a + c) ⇒ 2b4 – b2ac – a2c2 = 0 ...(iii) ⇒ (b2 – ac) (2b2 + ac) = 0 ⇒ b2 – ac = 0 or 2b2 + ac = 0 ⇒ \(\bigg(\frac{a+c}{2}\bigg)^2\) - ac = 0 b2 = \(-\frac{ac}{2}\) ⇒ (a – c)2 = 0 a,b, \(-\frac{c}{2}\) are in G.P. ⇒ 2b = 2c (From (i)) ⇒ b = c. |
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