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# I've been stuck on this problem for ages! Is it actually easy?

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A stick has a length of 5 units. The stick is then broken at two points, chosen at random. What is the probability that all three resulting pieces are longer than 1 unit?

Aug 5, 2020

#1
+25750
+4

A stick has a length of $$5$$ units. The stick is then broken at two points, chosen at random.

What is the probability that all three resulting pieces are longer than $$1$$ unit?

The probability that all three resulting pieces are longer than $$1$$ unit = $$\dfrac{ A_{\color{darkorange}orange} } {A}$$

$$\begin{array}{|rcll|} \hline H^2 + \left(\dfrac{S}{2}\right)^2 &=& S^2 \\ H^2 &=& S^2 -\dfrac{S^2}{4} \\ H^2 &=& \dfrac{3S^2}{4} \qquad \mathbf{H=\dfrac{S\sqrt{3}}{2}} \qquad (1) \\ S^2 &=& \dfrac{4H^2}{3} \\ S &=& \dfrac{2H}{\sqrt{3}} * \dfrac{\sqrt{3}}{\sqrt{3}} \\ \mathbf{S} &=& \mathbf{\dfrac{2H\sqrt{3}}{3}} \qquad (2) \\ \hline S &=& \dfrac{2H\sqrt{3}}{3} \quad | \quad H = 5 \\ \mathbf{S} &=& \mathbf{\dfrac{10\sqrt{3}}{3}} \\ \hline 2A &=& H*S \quad | \quad H=5,\ \mathbf{S=\dfrac{10\sqrt{3}}{3}} \\ \mathbf{2A} &=& \mathbf{\dfrac{50\sqrt{3}}{3}} \\ \hline \end{array}$$

$$\begin{array}{|rcll|} \hline \tan(30^\circ) &=& \dfrac{1}{x} \quad | \quad \tan(30^\circ) = \dfrac{1}{\sqrt{3}}\\ \dfrac{1}{\sqrt{3}} &=& \dfrac{1}{x} \\ \mathbf{ x } &=& \mathbf{\sqrt{3}} \\ \hline \mathbf{s} &=& \mathbf{S-2x} \quad | \quad \mathbf{S=\dfrac{10\sqrt{3}}{3}},\ \mathbf{ x =\sqrt{3}} \\ s &=& \dfrac{10\sqrt{3}}{3}-2\sqrt{3} \\ s &=& \dfrac{10\sqrt{3}}{3}-\dfrac{6\sqrt{3}}{3} \\ \mathbf{ s } &=& \mathbf{\dfrac{4\sqrt{3}}{3}} \\ \hline \mathbf{h^2 + \left(\dfrac{s}{2}\right)^2} &=& \mathbf{s^2} \\ h^2 &=& s^2 -\dfrac{s^2}{4} \\ h^2 &=& \dfrac{3s^2}{4} \\ \mathbf{h} &=& \mathbf{\dfrac{s\sqrt{3}}{2}}\quad | \quad \mathbf{ s =\dfrac{4\sqrt{3}}{3}} \\ h &=& \mathbf{\dfrac{\dfrac{4\sqrt{3}}{3}\sqrt{3}}{2}} \\ h &=& \dfrac{4}{2} \\ \mathbf{h} &=& \mathbf{2} \\ \hline 2A_{\color{darkorange}orange}&=& h*s \quad | \quad h=2,\ \mathbf{ s=\dfrac{4\sqrt{3}}{3}} \\ 2A_{\color{darkorange}orange} &=& 2*\dfrac{4\sqrt{3}}{3} \\ \mathbf{2A_{\color{darkorange}orange}} &=& \mathbf{\dfrac{8\sqrt{3}}{3}} \\ \hline \end{array}$$

$$\begin{array}{|rcll|} \hline \dfrac{ A_{\color{darkorange}orange} } {A} &=& \dfrac{ 2A_{\color{darkorange}orange} } {2A} \\\\ &=& \dfrac{ \dfrac{8\sqrt{3}}{3} } {\dfrac{50\sqrt{3}}{3}} \\\\ &=& \dfrac{8} {50} \\\\ \mathbf{ \dfrac{ A_{\color{darkorange}orange} } {A} } &=& \mathbf{ \dfrac{4}{25} } \\ \hline \end{array}$$

Aug 5, 2020

#1
+25750
+4

A stick has a length of $$5$$ units. The stick is then broken at two points, chosen at random.

What is the probability that all three resulting pieces are longer than $$1$$ unit?

The probability that all three resulting pieces are longer than $$1$$ unit = $$\dfrac{ A_{\color{darkorange}orange} } {A}$$

$$\begin{array}{|rcll|} \hline H^2 + \left(\dfrac{S}{2}\right)^2 &=& S^2 \\ H^2 &=& S^2 -\dfrac{S^2}{4} \\ H^2 &=& \dfrac{3S^2}{4} \qquad \mathbf{H=\dfrac{S\sqrt{3}}{2}} \qquad (1) \\ S^2 &=& \dfrac{4H^2}{3} \\ S &=& \dfrac{2H}{\sqrt{3}} * \dfrac{\sqrt{3}}{\sqrt{3}} \\ \mathbf{S} &=& \mathbf{\dfrac{2H\sqrt{3}}{3}} \qquad (2) \\ \hline S &=& \dfrac{2H\sqrt{3}}{3} \quad | \quad H = 5 \\ \mathbf{S} &=& \mathbf{\dfrac{10\sqrt{3}}{3}} \\ \hline 2A &=& H*S \quad | \quad H=5,\ \mathbf{S=\dfrac{10\sqrt{3}}{3}} \\ \mathbf{2A} &=& \mathbf{\dfrac{50\sqrt{3}}{3}} \\ \hline \end{array}$$

$$\begin{array}{|rcll|} \hline \tan(30^\circ) &=& \dfrac{1}{x} \quad | \quad \tan(30^\circ) = \dfrac{1}{\sqrt{3}}\\ \dfrac{1}{\sqrt{3}} &=& \dfrac{1}{x} \\ \mathbf{ x } &=& \mathbf{\sqrt{3}} \\ \hline \mathbf{s} &=& \mathbf{S-2x} \quad | \quad \mathbf{S=\dfrac{10\sqrt{3}}{3}},\ \mathbf{ x =\sqrt{3}} \\ s &=& \dfrac{10\sqrt{3}}{3}-2\sqrt{3} \\ s &=& \dfrac{10\sqrt{3}}{3}-\dfrac{6\sqrt{3}}{3} \\ \mathbf{ s } &=& \mathbf{\dfrac{4\sqrt{3}}{3}} \\ \hline \mathbf{h^2 + \left(\dfrac{s}{2}\right)^2} &=& \mathbf{s^2} \\ h^2 &=& s^2 -\dfrac{s^2}{4} \\ h^2 &=& \dfrac{3s^2}{4} \\ \mathbf{h} &=& \mathbf{\dfrac{s\sqrt{3}}{2}}\quad | \quad \mathbf{ s =\dfrac{4\sqrt{3}}{3}} \\ h &=& \mathbf{\dfrac{\dfrac{4\sqrt{3}}{3}\sqrt{3}}{2}} \\ h &=& \dfrac{4}{2} \\ \mathbf{h} &=& \mathbf{2} \\ \hline 2A_{\color{darkorange}orange}&=& h*s \quad | \quad h=2,\ \mathbf{ s=\dfrac{4\sqrt{3}}{3}} \\ 2A_{\color{darkorange}orange} &=& 2*\dfrac{4\sqrt{3}}{3} \\ \mathbf{2A_{\color{darkorange}orange}} &=& \mathbf{\dfrac{8\sqrt{3}}{3}} \\ \hline \end{array}$$

$$\begin{array}{|rcll|} \hline \dfrac{ A_{\color{darkorange}orange} } {A} &=& \dfrac{ 2A_{\color{darkorange}orange} } {2A} \\\\ &=& \dfrac{ \dfrac{8\sqrt{3}}{3} } {\dfrac{50\sqrt{3}}{3}} \\\\ &=& \dfrac{8} {50} \\\\ \mathbf{ \dfrac{ A_{\color{darkorange}orange} } {A} } &=& \mathbf{ \dfrac{4}{25} } \\ \hline \end{array}$$

heureka Aug 5, 2020
#3
+118665
0

That's pretty neat, heureka!!!!

I like this problem, even though I would have no clue as to the  solution!!!!

CPhill  Feb 20, 2021
#2
+113244
+1

Thanks Heureka,

I would have done this with a probability contour map.

Aug 5, 2020