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Mathematics

If x = 5 - 262\sqrt{6}, find the value of:

  1. x + 1x\dfrac{1}{x}

  2. x2+1x2x^2 +\dfrac{1}{x^2}

Rational Irrational Nos

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Answer

1. Given: x = 5 - 262\sqrt{6}

1x=1526\dfrac{1}{x} = \dfrac{1}{5 - 2\sqrt{6}}

1x=1×(5+26)(526)×(5+26)=5+2652(26)2=5+262524=5+26\Rightarrow \dfrac{1}{x} = \dfrac{1 \times (5 + 2\sqrt{6})}{(5 - 2\sqrt{6}) \times (5 + 2\sqrt{6})}\\[1em] = \dfrac{5 + 2\sqrt{6}}{5^2 - (2\sqrt{6})^2}\\[1em] = \dfrac{5 + 2\sqrt{6}}{25 - 24}\\[1em] = 5 + 2\sqrt{6}

Now, the value of x+1xx + \dfrac{1}{x} = (5+26)+(526)(5 + 2\sqrt{6}) + (5 - 2\sqrt{6})

= 5+26+5265 + 2\sqrt{6} + 5 - 2\sqrt{6}

= 10

Hence, the value of x+1xx + \dfrac{1}{x} = 10.

2. x+1xx + \dfrac{1}{x} = 10

Squaring both sides, we get

(x+1x)2=102x2+1x2+2×x×1x=100x2+1x2+2=100x2+1x2=1002x2+1x2=98\Rightarrow\Big(x + \dfrac{1}{x}\Big)^2 = 10^2\\[1em] \Rightarrow x^2 + \dfrac{1}{x^2} + 2\times x \times \dfrac{1}{x} = 100\\[1em] \Rightarrow x^2 + \dfrac{1}{x^2} + 2 = 100\\[1em] \Rightarrow x^2 + \dfrac{1}{x^2} = 100 - 2\\[1em] \Rightarrow x^2 + \dfrac{1}{x^2} = 98

Hence, the value of x2+1x2=98x^2 +\dfrac{1}{x^2} = 98.

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