Expand (5+2x)12\displaystyle (5+2x)^{\frac{1}{2}}(5+2x)21 in ascending powers of x x\,x up to and including x3x^3x3.
Write down the range of values of x x\,x for which the expansion is valid.
Use your expansion, with x=0.5x=0.5x=0.5, to find an approximation of the value of 6\sqrt{6}6. Give your answer to 3 decimal places.