A square pyramid has a base area of (25+7) cm2\left(2\sqrt{5} + 7\right)\text{ cm}^2 and a volume of (345+32) cm3\left(34\sqrt{5} + 32\right)\text{ cm}^3. Find the height of the pyramid in the form p+q5p + q\sqrt{5}, where pp and qq are integers.

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Worked solution

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1. Set up the equation for the volume of the pyramid and make $h$ the subject

Let the height of the pyramid be hh.
V=13×base area×hV = \frac{1}{3} \times \text{base area} \times h
345+32=13(25+7)h34\sqrt{5} + 32 = \frac{1}{3}(2\sqrt{5} + 7)h
h=3(345+32)7+25h = \frac{3(34\sqrt{5} + 32)}{7 + 2\sqrt{5}}
h=1025+967+25h = \frac{102\sqrt{5} + 96}{7 + 2\sqrt{5}}

2. Rationalise the denominator by multiplying by the conjugate

h=96+10257+25×7−257−25h = \frac{96 + 102\sqrt{5}}{7 + 2\sqrt{5}} \times \frac{7 - 2\sqrt{5}}{7 - 2\sqrt{5}}
h=(96+1025)(7−25)72−(25)2h = \frac{(96 + 102\sqrt{5})(7 - 2\sqrt{5})}{7^2 - (2\sqrt{5})^2}

3. Expand the numerator and simplify the expression

h=96(7)−96(25)+1025(7)−1025(25)49−20h = \frac{96(7) - 96(2\sqrt{5}) + 102\sqrt{5}(7) - 102\sqrt{5}(2\sqrt{5})}{49 - 20}
h=672−1925+7145−102029h = \frac{672 - 192\sqrt{5} + 714\sqrt{5} - 1020}{29}
h=−348+522529h = \frac{-348 + 522\sqrt{5}}{29}
h=−12+185h = -12 + 18\sqrt{5}

Answer

−12+185-12 + 18\sqrt{5}

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