Question

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IF x=√a+2b+√a-2b divide by√a+2b-√a-2b

prove that bx²-ax+b=0

Answer 1

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Answer 2

$\bf \underline{Given-}$

$\sf{x = \frac{ \sqrt{a + 2b} + \sqrt{a - 2b} }{ \sqrt{a + 2b} - \sqrt{a - 2b} } }$

$\bf \underline{To\: find-}$

$\sf{prove \: that : \: {bx}^{2} - ax + b = 0 }$

$\bf \underline{Solution-}$

$\textsf{We have,}$

$\sf{x = \frac{ \sqrt{a + 2b} + \sqrt{a - 2b} }{ \sqrt{a + 2b} - \sqrt{a - 2b} } }$

$\textsf{The denominator is : √(a+2b) - √(a-2b)}$

$\textsf{We know that}$

$\textsf{The rationalising factor of : √(p + q) - √(p-q) = √(p+q) + √(p-q).}$

$\textsf{Therefore, the rationalising factor of: √(a+2b) - √(a-2b) = √(a+2b) + √(a-2b).}$

$\textsf{On, rationalising the denominator,we get}$

$\sf{x = \frac{ \sqrt{a + 2b} + \sqrt{a - 2b} }{ \sqrt{a + 2b} - \sqrt{a - 2b} } \times\frac{ \sqrt{a + 2b} + \sqrt{a - 2b} }{ \sqrt{a + 2b} + \sqrt{a - 2b} } }$

$\sf{x = \frac{ (\sqrt{a + 2b} + \sqrt{a - 2b})( \sqrt{a + 2b} + \sqrt{a - 2b}) }{( \sqrt{a + 2b} - \sqrt{a - 2b})( \sqrt{a + 2b} + \sqrt{a - 2b} )} }$

$\sf{x = \frac{ (\sqrt{a + 2b} + \sqrt{a - 2b} {)}^{2} }{( \sqrt{a + 2b} - \sqrt{a - 2b})( \sqrt{a + 2b} + \sqrt{a - 2b} )} }$

$\textsf{★ Now, comparing the denominator with (a-b)(a+b), we get}$

$\sf{ \: \: \: \: \: a = \sqrt{a + 2b} \: and \: b = \sqrt{a - 2b} }$

$\textsf{Using identity (a+b)(a-b) = a²-b², we get}$

$\sf{x = \frac{ (\sqrt{a + 2b} + \sqrt{a - 2b} {)}^{2} }{( \sqrt{a + 2b} {)}^{2} - (\sqrt{a - 2b} {)}^{2} } }$

$\sf{x = \frac{ (\sqrt{a + 2b} + \sqrt{a - 2b} {)}^{2} }{a + 2b - (a + 2b) } }$

$\sf{x = \frac{ a + 2b + a - 2b + 2 \sqrt{ {a}^{2} - {4b}^{2} } }{a + 2b - (a + 2b) } }$

$\Rightarrow\sf{x = \frac{a + \sqrt{ {a}^{2} - {4b}^{2} } }{2b} }$

$\Rightarrow\sf{2bx =a + \sqrt{ {a}^{2} - {4b}^{2} } }$

$\Rightarrow\sf{2bx - a = \sqrt{ {a}^{2} - {4b}^{2} } }$

$\textsf{Squaring on both sides,we get}$

$\sf{(2bx - a {)}^{2} = {a}^{2} - 4 {b}^{2} }$

$\Rightarrow\sf{ {4b}^{2} {x}^{2} + \cancel{{a}^{2}} - 4abx - \cancel{ {a}^{2}} + {4b}^{2} =0}$

$\Rightarrow\sf{ {4b}^{2} {x}^{2} - 4abx + {4b}^{2} =0 \: \: \Rightarrow\sf{4( {b}^{2} {x}^{2} - abx + {4b}^{2} ) =0} }$

$\Rightarrow\sf{{b}^{2} {x}^{2} - abx + {4b}^{2} =0 \: \: \: \: \: \Rightarrow\sf{b( {b}{x}^{2} - ax + {b} ) =0} }$

$\Rightarrow\sf{{b}{x}^{2} - ax + {b} =0}$

$\bf \underline{Hence, proved.}$