Question Number 177031 by HeferH last updated on 29/Sep/22
Answered by mr W last updated on 30/Sep/22
Commented by mr W last updated on 30/Sep/22
$$\mathrm{cos}^{−\mathrm{1}} \frac{\mathrm{5}}{\mathrm{2}{R}}−\mathrm{cos}^{−\mathrm{1}} \frac{\mathrm{6}}{\mathrm{2}{R}}=\mathrm{30}° \\ $$$$\frac{\mathrm{5}}{\mathrm{2}{R}}×\frac{\mathrm{6}}{\mathrm{2}{R}}+\sqrt{\left(\mathrm{1}−\frac{\mathrm{25}}{\mathrm{4}{R}^{\mathrm{2}} }\right)\left(\mathrm{1}−\frac{\mathrm{36}}{\mathrm{4}{R}^{\mathrm{2}} }\right)}=\frac{\sqrt{\mathrm{3}}}{\mathrm{2}} \\ $$$$\left(\mathrm{1}−\frac{\mathrm{25}}{\mathrm{4}{R}^{\mathrm{2}} }\right)\left(\mathrm{1}−\frac{\mathrm{36}}{\mathrm{4}{R}^{\mathrm{2}} }\right)=\left(\frac{\sqrt{\mathrm{3}}}{\mathrm{2}}−\frac{\mathrm{30}}{\mathrm{4}{R}^{\mathrm{2}} }\right)^{\mathrm{2}} \\ $$$$\mathrm{1}−\frac{\mathrm{61}}{\mathrm{4}{R}^{\mathrm{2}} }+\frac{\mathrm{25}×\mathrm{36}}{\left(\mathrm{4}{R}^{\mathrm{2}} \right)^{\mathrm{2}} }=\frac{\mathrm{3}}{\mathrm{4}}−\frac{\mathrm{30}\sqrt{\mathrm{3}}}{\mathrm{4}{R}^{\mathrm{2}} }+\frac{\mathrm{30}^{\mathrm{2}} }{\left(\mathrm{4}{R}^{\mathrm{2}} \right)^{\mathrm{2}} } \\ $$$$\frac{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}}{{R}^{\mathrm{2}} }=\mathrm{1} \\ $$$$\Rightarrow{R}=\sqrt{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}} \\ $$$${CD}=\sqrt{{R}^{\mathrm{2}} −\left(\frac{\mathrm{5}}{\mathrm{2}}\right)^{\mathrm{2}} }=\sqrt{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}−\frac{\mathrm{25}}{\mathrm{4}}}=\frac{\sqrt{\mathrm{219}−\mathrm{120}\sqrt{\mathrm{3}}}}{\mathrm{2}} \\ $$$${CB}={R}−{r} \\ $$$${DE}=\frac{{r}}{\mathrm{tan}\:\mathrm{15}°}−\frac{\mathrm{5}}{\mathrm{2}}=\left(\mathrm{2}+\sqrt{\mathrm{3}}\right){r}−\frac{\mathrm{5}}{\mathrm{2}} \\ $$$${CD}−{BE}=\frac{\sqrt{\mathrm{219}−\mathrm{120}\sqrt{\mathrm{3}}}}{\mathrm{2}}−{r} \\ $$$$\left(\frac{\sqrt{\mathrm{219}−\mathrm{120}\sqrt{\mathrm{3}}}}{\mathrm{2}}−{r}\right)^{\mathrm{2}} +\left(\left(\mathrm{2}+\sqrt{\mathrm{3}}\right){r}−\frac{\mathrm{5}}{\mathrm{2}}\right)^{\mathrm{2}} =\left(\sqrt{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}}−{r}\right)^{\mathrm{2}} \\ $$$$\frac{\mathrm{219}−\mathrm{120}\sqrt{\mathrm{3}}}{\mathrm{4}}−\sqrt{\mathrm{219}−\mathrm{120}\sqrt{\mathrm{3}}}{r}+\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{\mathrm{2}} {r}^{\mathrm{2}} −\mathrm{5}\left(\mathrm{2}+\sqrt{\mathrm{3}}\right){r}+\frac{\mathrm{25}}{\mathrm{4}}=\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}−\mathrm{2}\sqrt{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}}{r} \\ $$$$\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{\mathrm{2}} {r}=\sqrt{\mathrm{219}−\mathrm{120}\sqrt{\mathrm{3}}}+\mathrm{5}\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)−\mathrm{2}\sqrt{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}} \\ $$$$\Rightarrow{r}=\frac{\sqrt{\mathrm{219}−\mathrm{120}\sqrt{\mathrm{3}}}+\mathrm{5}\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)−\mathrm{2}\sqrt{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}}}{\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{\mathrm{2}} } \\ $$$$\:\:\:\:\:\:\:\approx\mathrm{1}.\mathrm{147828} \\ $$
Commented by mr W last updated on 30/Sep/22
$${an}\:{other}\:{way}\:{to}\:{find}\:{R}: \\ $$$${R}=\frac{\sqrt{{a}^{\mathrm{2}} +{b}^{\mathrm{2}} −\mathrm{2}{ab}\:\mathrm{cos}\:\theta}}{\mathrm{2}\:\mathrm{sin}\:\theta} \\ $$$$\:\:\:\:=\frac{\sqrt{\mathrm{5}^{\mathrm{2}} +\mathrm{6}^{\mathrm{2}} −\mathrm{2}×\mathrm{5}×\mathrm{6}×\frac{\sqrt{\mathrm{3}}}{\mathrm{2}}}}{\mathrm{2}×\frac{\mathrm{1}}{\mathrm{2}}} \\ $$$$\:\:\:\:=\sqrt{\mathrm{61}−\mathrm{30}\sqrt{\mathrm{3}}} \\ $$
Commented by Tawa11 last updated on 02/Oct/22
$$\mathrm{Great}\:\mathrm{sir} \\ $$