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Arrange the following in descending order:

A. 401−399\sqrt{401}-\sqrt{399}

B. 301−299\sqrt{301}-\sqrt{299}

C. 101−99\sqrt{101}-\sqrt{99}

D. 201−199\sqrt{201}-\sqrt{199}

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 4

Each option is of the form bigger−smaller\sqrt{\text{bigger}}-\sqrt{\text{smaller}}, which is painful to compare directly. Rationalise each one by multiplying and dividing by its conjugate, using the identity (A−B)(A+B)=A2−B2(A-B)(A+B)=A^2-B^2.

For any number xx,

x+1−x−1=(x+1)−(x−1)x+1+x−1=2x+1+x−1\sqrt{x+1}-\sqrt{x-1}=\dfrac{(x+1)-(x-1)}{\sqrt{x+1}+\sqrt{x-1}}=\dfrac{2}{\sqrt{x+1}+\sqrt{x-1}}


OptionAfter rationalisingSize of denominator
A2401+399\dfrac{2}{\sqrt{401}+\sqrt{399}}largest
B2301+299\dfrac{2}{\sqrt{301}+\sqrt{299}}third
C2101+99\dfrac{2}{\sqrt{101}+\sqrt{99}}smallest
D2201+199\dfrac{2}{\sqrt{201}+\sqrt{199}}second smallest

Every option now has the same numerator 2, so the smaller the denominator, the bigger the value.

Descending order:

C>D>B>AC>D>B>A

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