Question Number 146457 by mathdanisur last updated on 13/Jul/21
Commented by mathdanisur last updated on 13/Jul/21
$${find}\:\:{x}=? \\ $$
Answered by gsk2684 last updated on 13/Jul/21
$${let}\:\angle{BAC}\:=\theta \\ $$$${using}\:{cosine}\:{rule}\: \\ $$$$\mathrm{7}^{\mathrm{2}} ={x}^{\mathrm{2}} +\mathrm{2}^{\mathrm{2}} −\mathrm{2}.{x}.\mathrm{2}.\mathrm{cos}\:\left(\mathrm{90}^{\mathrm{0}} +\theta\right) \\ $$$$\mathrm{49}={x}^{\mathrm{2}} +\mathrm{4}+\mathrm{4}{x}\:\mathrm{sin}\:\theta \\ $$$$\mathrm{45}={x}^{\mathrm{2}} +\mathrm{4}{x}\left(\frac{{x}}{\mathrm{23}}\right) \\ $$$$\mathrm{45}×\mathrm{23}=\mathrm{23}{x}^{\mathrm{2}} +\mathrm{4}{x}^{\mathrm{2}} \\ $$$${x}^{\mathrm{2}} =\frac{\mathrm{45}×\mathrm{23}}{\mathrm{27}}=\frac{\mathrm{115}}{\mathrm{3}} \\ $$$${x}=\sqrt{\frac{\mathrm{115}}{\mathrm{3}}} \\ $$
Commented by mathdanisur last updated on 13/Jul/21
$${thank}\:{you}\:{Ser}\:{cool} \\ $$
Answered by mr W last updated on 15/Jul/21
$${x}^{\mathrm{2}} =\mathrm{2}^{\mathrm{2}} +\mathrm{7}^{\mathrm{2}} −\mathrm{2}×\mathrm{2}×\mathrm{7}\:\mathrm{cos}\:{D}\:\:\:\:…\left({i}\right) \\ $$$$\mathrm{23}^{\mathrm{2}} −{x}^{\mathrm{2}} =\mathrm{25}^{\mathrm{2}} +\mathrm{7}^{\mathrm{2}} −\mathrm{2}×\mathrm{25}×\mathrm{7}\:\mathrm{cos}\:{D}\:\:\:…\left({ii}\right) \\ $$$$\left({i}\right)×\mathrm{25}−\left({ii}\right)×\mathrm{2}: \\ $$$$\mathrm{25}{x}^{\mathrm{2}} −\mathrm{2}×\mathrm{23}^{\mathrm{2}} +\mathrm{2}{x}^{\mathrm{2}} =\mathrm{25}\left(\mathrm{2}^{\mathrm{2}} +\mathrm{7}^{\mathrm{2}} \right)−\mathrm{2}\left(\mathrm{25}^{\mathrm{2}} +\mathrm{7}^{\mathrm{2}} \right) \\ $$$$\mathrm{27}{x}^{\mathrm{2}} =\mathrm{45}×\mathrm{23} \\ $$$${x}=\frac{\sqrt{\mathrm{345}}}{\mathrm{3}} \\ $$
Commented by mathdanisur last updated on 19/Jul/21
$${thank}\:{you}\:{Ser},\:{but}\:{answer}:\:\:\frac{\sqrt{\mathrm{115}}}{\mathrm{3}} \\ $$
Commented by mr W last updated on 20/Jul/21
$${you}\:{should}\:{check}\:{before}\:{you}\:{say}! \\ $$$${where}\:{is}\:{wrong}\:{in}\:{my}\:{working}? \\ $$$${my}\:{answer}\:{is}\:\frac{\sqrt{\mathrm{345}}}{\mathrm{3}}.\:{if}\:{you}\:{mean}\:{the} \\ $$$${answer}\:{is}\:\sqrt{\frac{\mathrm{115}}{\mathrm{3}}},\:{then}\:\frac{\sqrt{\mathrm{345}}}{\mathrm{3}}\:{is}\:{also} \\ $$$${right}. \\ $$