Simplification of surd expression

CAT 2024 Slot 3 · QA · Easy · Number Systems

If (a+b3)2=52+303(a + b\sqrt{3})^2 = 52 + 30\sqrt{3}, where aa and bb are natural numbers, then a+ba + b equals

  1. A.

    7

  2. B.

    10

  3. C.

    8

  4. D.

    9

Answer

C

Explanation

Expanding the given equation: (a+b3)2=a2+3b2+2ab3=52+303(a + b\sqrt{3})^2 = a^2 + 3b^2 + 2ab\sqrt{3} = 52 + 30\sqrt{3}

Comparing rational and irrational parts: 2ab=30    ab=152ab = 30 \implies ab = 15 a2+3b2=52a^2 + 3b^2 = 52

Since aa and bb are natural numbers, the possible pairs for (a,b)(a, b) are:

  • If a=5,b=3a = 5, b = 3: a2+3b2=52+3(32)=25+27=52a^2 + 3b^2 = 5^2 + 3(3^2) = 25 + 27 = 52. This satisfies both conditions.

Hence, a=5a = 5 and b=3b = 3, giving a+b=5+3=8a + b = 5 + 3 = 8.

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