Question Number 137363 by liberty last updated on 02/Apr/21 | ||
$$ \\ $$ As illustrated, the rectangle has an area of 1, and E is the midpoint of AD. BF is one third of AB. What is the area of the shadow?\\n | ||
Commented byliberty last updated on 02/Apr/21 | ||
Answered by mr W last updated on 02/Apr/21 | ||
Commented bymr W last updated on 02/Apr/21 | ||
$${EJ}=\frac{{AF}}{\mathrm{2}}={BF} \\ $$ $$\Rightarrow{BG}={GE} \\ $$ $${EJ}=\frac{{AF}}{\mathrm{2}}=\frac{\mathrm{1}}{\mathrm{2}}×\frac{\mathrm{2}}{\mathrm{3}}{AB}=\frac{{AB}}{\mathrm{3}}=\frac{{DC}}{\mathrm{3}} \\ $$ $$\Rightarrow{EI}=\frac{{IC}}{\mathrm{3}}=\frac{{EC}}{\mathrm{4}} \\ $$ $${FK}=\frac{{AE}}{\mathrm{3}}=\frac{{BC}}{\mathrm{6}} \\ $$ $$\Rightarrow{BK}=\mathrm{6}×{HK}=\frac{\mathrm{6}}{\mathrm{7}}×{BK}=\frac{\mathrm{6}}{\mathrm{7}}×\frac{{BE}}{\mathrm{3}}=\frac{\mathrm{2}}{\mathrm{7}}×{BE} \\ $$ $$\Delta{BHC}=\frac{\mathrm{2}}{\mathrm{7}}×\Delta{BEC} \\ $$ $$\Delta{EGI}=\frac{\mathrm{1}}{\mathrm{4}}×\Delta{EGC}=\frac{\mathrm{1}}{\mathrm{8}}×\Delta{BEC} \\ $$ $${shados}=\left(\mathrm{1}−\frac{\mathrm{2}}{\mathrm{7}}−\frac{\mathrm{1}}{\mathrm{8}}\right)×\Delta{BEC} \\ $$ $$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:=\frac{\mathrm{33}}{\mathrm{56}}×\Delta{BEC} \\ $$ $$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:=\frac{\mathrm{33}}{\mathrm{56}}×\frac{\mathrm{1}}{\mathrm{2}}=\frac{\mathrm{\color{mathred}{3}\color{mathred}{3}}}{\mathrm{\color{mathred}{1}\color{mathred}{1}\color{mathred}{2}}} \\ $$ | ||
Answered by EDWIN88 last updated on 02/Apr/21 | ||
$$\mathrm{we}\:\mathrm{can}\:\mathrm{make}\:\mathrm{simpler}\:\mathrm{by}\:\mathrm{considering}\:\mathrm{this} \\ $$ $$\mathrm{a}\:\mathrm{12}×\mathrm{12}\:\mathrm{square}\:\mathrm{with}\:\mathrm{B}\:\mathrm{at}\:\mathrm{origin}.\:\mathrm{We}\:\mathrm{have} \\ $$ $$\mathrm{to}\:\mathrm{remember}\:\mathrm{to}\:\mathrm{divide}\:\mathrm{by}\:\mathrm{12}^{\mathrm{2}} \:\mathrm{at}\:\mathrm{the}\:\mathrm{end}. \\ $$ $$\mathrm{We}\:\mathrm{get}\:\mathrm{coordinates}\:\mathrm{B}\left(\mathrm{0},\mathrm{0}\right)\:,\mathrm{A}\left(\mathrm{0},\mathrm{12}\right),\mathrm{F}\left(\mathrm{0},\mathrm{4}\right) \\ $$ $$\mathrm{E}\left(\mathrm{6},\mathrm{12}\right),\:\mathrm{D}\left(\mathrm{12},\mathrm{12}\right),\mathrm{C}\left(\mathrm{12},\mathrm{0}\right).\:\mathrm{we}\:\mathrm{need}\:\mathrm{to}? \\ $$ $$\mathrm{calculate}\:\mathrm{G},\mathrm{H},\mathrm{I}.\:\mathrm{G}\:\mathrm{is}\:\mathrm{the}\:\mathrm{meet}\:\mathrm{if}\:\mathrm{BE}\:\mathrm{and}\:\mathrm{DF} \\ $$ $$\:\Rightarrow\:\mathrm{G}\left(\mathrm{3},\mathrm{6}\right);\:\mathrm{H}\left(\frac{\mathrm{12}}{\mathrm{7}},\frac{\mathrm{24}}{\mathrm{7}}\right)\:;\:\mathrm{I}\left(\frac{\mathrm{15}}{\mathrm{2}},\:\mathrm{9}\right).\: \\ $$ $$\mathrm{The}\:\mathrm{shadow}\:\mathrm{area}\:\mathrm{is}\: \\ $$ $$=\:\frac{\mathrm{1}}{\mathrm{2}}\:\mid\:\begin{vmatrix}{\mathrm{12}\:\:\:\:\mathrm{0}}\\{\frac{\mathrm{12}}{\mathrm{7}}\:\:\:\:\frac{\mathrm{24}}{\mathrm{7}}}\end{vmatrix}+\begin{vmatrix}{\frac{\mathrm{12}}{\mathrm{7}}\:\:\:\:\frac{\mathrm{24}}{\mathrm{7}}}\\{\:\mathrm{3}\:\:\:\:\:\:\:\mathrm{6}}\end{vmatrix}+\begin{vmatrix}{\mathrm{3}\:\:\:\:\mathrm{6}}\\{\frac{\mathrm{15}}{\mathrm{2}}\:\:\mathrm{9}}\end{vmatrix}+\begin{vmatrix}{\frac{\mathrm{15}}{\mathrm{2}}\:\:\:\mathrm{9}}\\{\mathrm{12}\:\:\:\mathrm{0}}\end{vmatrix}\mid \\ $$ $$=\frac{\mathrm{1}}{\mathrm{2}}\mid\frac{\mathrm{288}}{\mathrm{7}}\:+\:\mathrm{0}\:−\mathrm{18}−\mathrm{108}\:\mid \\ $$ $$=\frac{\mathrm{1}}{\mathrm{2}}.\:\frac{\mathrm{594}}{\mathrm{7}}\:=\:\frac{\mathrm{297}}{\mathrm{7}}\:.\mathrm{Therefore}\:\mathrm{we}\:\mathrm{get}\:\mathrm{the} \\ $$ $$\mathrm{shadow}\:\mathrm{area}\:\mathrm{is}\:\frac{\mathrm{297}}{\mathrm{7}×\mathrm{144}}\:=\:\frac{\mathrm{33}}{\mathrm{112}}\: \\ $$ $$ \\ $$ | ||