Area of a triangle at Plane Geometry Forum

In square $ABCD$, $E$ is the midpoint of side $\overline{AB}$ and $F$ is a point of side $\overline{AD}$ such that $F$ is twice as near from $D$ as from $A$. $G$ is the intersection of the line segments $\overline{DE}$ and $\overline{CF}$. If $AB = 1\text{ cm}$, find the area of $\triangle CDG$. There are several ways to solve this problem, please show your solutions.

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Area of Triangle

alidoandrei28 Tue, 06/08/2021 - 15:48

I used analytic geometry to get the equation of the lines DE and CF. Thus, DE: (0,0) (0.5,0.5) equation: y = x FC: (0,1/3) (1,0) equation: y = 1/3 - (1/3)x Then, I get the point of intersection; (0.25,0.25) After that, I used integral calculus to solve for the area using horizontal strip. $[(1-3y) - y]dy from 0 to 0.25 = 0.125 Thus, the area of triangle CDG = 0.125 cm^2 -please correct me if I'm wrong. Thanks!

Angkol-Awin Wed, 06/09/2021 - 12:57

alidoandrei28 Please Check again your coordinates of line DE: (0,0) , must be (0.5,1).

Engr Jaydee Wed, 06/16/2021 - 15:49

Hi alidoandrei28, The coordinates of E should be (1/2, 1) instead of (1/2, 1/2). The rest of the solutions will be correct if this is considered. :)

ConradoJr Tue, 06/08/2021 - 16:41

Yes. Using the area of triangle for 2 sides and included angle, or Heron's formula for three sides. The answer is 0.14 cm2. Solution: Square ABCD, given 1cm for each side Line EB=Line EA=1/2cm Line FD=1/3cm Line AF=2/3cm Right Triangle BCE: By Pythagorean theorem, Line CE=square root of 1.25 or 1.12 (2 decimal place). Using tangent function, the other 2 angle (C & E) could be determined and that is 26.6 deg. and 63.4 deg. respectively. Right Triangle ADE: the same procedure as in triangle BCE to establish angle E & D. Right Triangle CDF: the same procedure as in triangle BCE & ADE to establish angle C. Triangle CEG: Angle E=180-63.4-63.4= 53.2; Angle C=90-26.6-18.4= 45; Angle G=180-98.2= 81.8; Now using sine law, line CG & EG could be solved. Triangle CDG: 3 interior angle already been established (Angle C, D & G = 18.4, 63.4 & 98.2 respectively). Hence, area of triangle CDG could be solved using area of triangle as mentioned above.

Engr Jaydee Wed, 06/16/2021 - 20:15

Hi ConradoJr! The answer and solution is correct, although I think it can shortened further. You can find angles EDC and FCD by using the tangent function, and then proceed to getting the area of triangle CDG using the formula $$A=\frac{a^2 \sin \beta \sin \gamma}{2 \sin \alpha}$$