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The quadratic equation whose one root is 3 + √5 is 

A) x2 – 6x – 4 = 0 

B) x2 + 6x + 4 = 0 

C) x2 – 6x + 4 = 0 

D) x2 + 6x + 5 = 0

2 Answers

+1 vote
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Best answer

Correct option is (C) x2 – 6x + 4 = 0

If \(3+\sqrt{5}\) is one root of quadratic equation.

Then \(3-\sqrt{5}\) is other root of that quadratic equation.

\(\therefore\) Required quadratic equation is

\((x-(3+\sqrt5))(x-(3-\sqrt5))=0\)

\(\Rightarrow((x-3)-\sqrt5)((x-3)+\sqrt5)=0\)

\(\Rightarrow(x-3)^2-(\sqrt5)^2=0\)    \((\because(a-b)(a+b)=a^2-b^2)\)

\(\Rightarrow x^2-6x+9-5=0\)      \((\because(a-b)^2=a^2-2ab+b^2)\)

\(\Rightarrow x^2-6x+4=0\)

+1 vote
by (41.1k points)

Correct option is C) x2 – 6x + 4 = 0

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