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If 7+2511+35\frac{7 + 2\sqrt{5}}{11 + 3\sqrt{5}}11+35​7+25​​  = a + b5\sqrt{5}5​, then:​
Question

If 7+2511+35\frac{7 + 2\sqrt{5}}{11 + 3\sqrt{5}}  = a + b5\sqrt{5}, then:​

A.

a = 1 ; b = 5\sqrt{5}​​

B.

a = 5;b=1\sqrt{5} ; b = 1​​

C.

a=116;b=4776a = \frac{1}{16} ; b = \frac{47}{76}​​

D.

a=4776;b=176a = \frac{47}{76} ; b = \frac{1}{76}​​

Correct option is D

Given:

7+2511+35\frac{7 + 2\sqrt{5}}{11 + 3\sqrt{5}} = a + b5\sqrt{5}

Solution:

To rationalize the denominator, we multiply by the conjugate of the denominator.

(7+25)(1135)(11+35)(1135)=>7×11+7×(35)+25×11+25×(35)112(35)2=>77215+2253012145=>47+576=>4776+1765\begin{aligned}& \frac{(7+2 \sqrt{5})(11-3 \sqrt{5})}{(11+3 \sqrt{5})(11-3 \sqrt{5})} \\& \Rightarrow \frac{7 \times 11+7 \times(-3 \sqrt{5})+2 \sqrt{5} \times 11+2 \sqrt{5} \times(-3 \sqrt{5})}{11^2-(3 \sqrt{5})^2} \\& \Rightarrow \frac{77-21 \sqrt{5}+22 \sqrt{5}-30}{121-45} \\& \Rightarrow \frac{47+\sqrt{5}}{76} \\& \Rightarrow \frac{47}{76}+\frac{1}{76} \sqrt{5}\end{aligned}    

Comparing with a + b5\sqrt{5} 

a = 4776\frac{47}{76} , b =176\frac{1}{76}​​

Thus, the correct answer is (D)

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