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The arithmetic mean of two numbers exceeds their geometric mean by 2 and the geometric mean exceeds their harmonic mean by 1.6. What are the two numbers?1. 16, 42. 81, 93. 256, 164. 625, 25 |
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Answer» Correct Answer - Option 1 : 16, 4 Concept: Let the two numbers as a and b. A.M. = \(\rm \dfrac {a+b}{2}\) G.M. = \(\rm\sqrt{ab}\) Relation between AM and GM: (G.M.)2 = (A.M.)(H.M.)
Calculations: Given, the arithmetic mean of two numbers exceeds their geometric mean by 2 and the geometric mean exceeds their harmonic mean by 1.6. ⇒ A.M. = G.M. + 2 ....(1) and G.M. = H.M. + 1.6 We know that, (G.M.)2 = (A.M.)(H.M.) ⇒ (G.M.)2 = (G.M. + 2)(G.M. - 1.6) ⇒ (G.M.)2 = (G.M.)2 + 0.4 G.M. - 3.2 ⇒ 0.4 G.M. = 3.2 ⇒ G.M. = 8 ....(2) From equation (1), we get ⇒A.M. = 10 ....(3) Consider, the two numbers as a and b. then, G.M. = \(\rm\sqrt{ab}\) and A.M. = \(\rm \dfrac {a+b}{2}\) From equation (2) and (3), we get ⇒ \(\rm\sqrt{ab}\) = 8 and \(\rm \dfrac {a+b}{2}\) = 10 ⇒ ab = 64 and a + b = 20 ⇒ a (20 - a) = 64 ⇒ a2 - 20a - 64 = 0 ⇒ a =16 or a = 4 ⇒ b = 4 or b = 16 Hence, the two numbers are 16 and 4. Hence, The arithmetic mean of two numbers exceeds their geometric mean by 2 and the geometric mean exceeds their harmonic mean by 1.6. the two numbers are 16 and 4. |
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