Without using a calculator, find the values of the integers aa and bb for which the solution of the equation xackslash√18−ackslash√72=xackslash√3xackslash√{18} - ackslash√{72} = xackslash√{3} is a+2b5\frac{a + 2\sqrt{b}}{5}.

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Worked solution

1

1. Rearrange the equation to group terms involving $x$ and simplify the surds.

x18−x3=72x\sqrt{18} - x\sqrt{3} = \sqrt{72}
x(32−3)=62x(3\sqrt{2} - \sqrt{3}) = 6\sqrt{2}

2. Isolate $x$.

x=6232−3x = \frac{6\sqrt{2}}{3\sqrt{2} - \sqrt{3}}

3. Rationalise the denominator by multiplying the numerator and denominator by the conjugate.

x=62(32+3)(32−3)(32+3)x = \frac{6\sqrt{2}(3\sqrt{2} + \sqrt{3})}{(3\sqrt{2} - \sqrt{3})(3\sqrt{2} + \sqrt{3})}

4. Expand and simplify the expression.

x=18(2)+66(32)2−(3)2x = \frac{18(2) + 6\sqrt{6}}{(3\sqrt{2})^2 - (\sqrt{3})^2}
x=36+6618−3x = \frac{36 + 6\sqrt{6}}{18 - 3}
x=36+6615x = \frac{36 + 6\sqrt{6}}{15}

5. Simplify the fraction to match the given form and identify $a$ and $b$.

Divide numerator and denominator by 3:
x=12+265x = \frac{12 + 2\sqrt{6}}{5}
Comparing with a+2b5\frac{a + 2\sqrt{b}}{5}, we have a=12a = 12 and b=6b = 6.

Answer

a=12a = 12, b=6b = 6

AI analysis — may contain mistakes.

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