1.

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

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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