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A concise way to get both answers is to place the square in a convenient coordinate system and solve for the coordinates of E and F. A workable choice is
A=(5,5),\quad B=(0,5),\quad C=(0,0),\quad D=(5,0),
so that E lies on the left edge BC and F lies on the bottom edge CD. One may set
E=(0,e),\quad F=(f,0),
with 0<e<5 and 0<f<5.
1. Use \angle BEA=62^\circ to find E.
The vectors
\overrightarrow{EB} = B - E = (0,5)-(0,e) = (0,5-e),
\quad
\overrightarrow{EA} = A - E = (5,5)-(0,e) = (5,5-e),
must make a 62^\circ angle. In particular,
\[
\cos 62^\circ
\;=\;
\frac{\overrightarrow{EB}\cdot \overrightarrow{EA}}{\lVert EB\rVert\,\lVert EA\rVert}
\frac{(5-e)^2}{(5-e)\sqrt{5^2+(5-e)^2}}
\;=\;
\frac{5-e}{\sqrt{25+(5-e)^2}}.
\]
Solving numerically gives 5-e\approx 2.66, hence e\approx 2.34.
2. Use \angle EAF=45^\circ to find F.
Now \overrightarrow{AE}=(0-e,2.34-5) and \overrightarrow{AF}=(f-5,-5) must make a 45^\circ angle. Imposing
\[
\cos 45^\circ
\;=\;
\frac{\overrightarrow{AE}\cdot \overrightarrow{AF}}{\lVert AE\rVert\,\lVert AF\rVert},
\]
one finds f\approx 3.47.
3. Locate \angle 1 at F.
From the diagram, \angle 1 is the angle at F between the segments FE and FD. In coordinates,
\overrightarrow{FE} \;=\; E-F = (0,2.34)-(3.47,0)=( -3.47,\,2.34),\quad
\overrightarrow{FD} \;=\; D-F = (5,0)-(3.47,0)=(1.53,\,0).
A straightforward dot‐product computation shows
\[
\cos(\angle 1)
\;=\;
\frac{\overrightarrow{FE}\cdot \overrightarrow{FD}}
{\lVert FE\rVert\,\lVert FD\rVert}
\;\approx\;
-0.829,
\]
giving
\angle 1 \;\approx\; 146^\circ.
4. Perimeter of \triangle CEF.
The vertices are
C=(0,0),\quad E=(0,2.34),\quad F=(3.47,0),
so the side lengths are
CE=2.34,\quad
CF=3.47,\quad
EF=\sqrt{(-3.47)^2 + 2.34^2}\,\approx\,4.19.
Their sum is very close to 2.34 + 3.47 + 4.19 \approx 10.
Hence the answers are
\boxed{\angle 1 \approx 146^\circ \quad\text{and}\quad
\text{perimeter of }\triangle CEF \approx 10\text{ cm.}}
公開 最後更新: 2025-01-27 09:28:59 AM