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Let SNF be my path to cross the field. Let SN = x \; \text{km}, then NM =1-x\; \text{km}. Using Pythogaras' theorem for the right angled triangle NMF: NF^2 = 1 + (1-x)^2 = x^2 -2x + 2. The total time taken T is given by T = {x\over 10} +
{(x^2 - 2x + 2)^{1/2}\over 6} By differentiation {dT\over dx} =
{1\over 10} + {2x-2\over 12(x^2 -2x + 2)^{1/2}}. For a maximum or
minimum time this derivative is zero, so that
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Generalise the sum of a GP by using derivatives to make the coefficients into powers of the natural numbers.
What is the longest stick that can be carried horizontally along a narrow corridor and around a right-angled bend?
Find all the turning points of y=x^{1/x} for x>0 and decide whether each is a maximum or minimum. Give a sketch of the graph.