1. Statement of the problem: Find the area and centroid of the quadrilateral with vertices $D(5,4)$, $E(8,2)$, $F(7,-2)$, and $G(4,-4)$.
2. Formulas and rules used: Use the shoelace formula for polygon area.
$$A=\tfrac12\left|\sum_{i=0}^{n-1}(x_i y_{i+1}-x_{i+1} y_i)\right|$$
Also use the centroid formulas for a polygon given a signed area $A$.
$$C_x=\frac{1}{6A}\sum_{i=0}^{n-1}(x_i+x_{i+1})(x_i y_{i+1}-x_{i+1} y_i)$$
$$C_y=\frac{1}{6A}\sum_{i=0}^{n-1}(y_i+y_{i+1})(x_i y_{i+1}-x_{i+1} y_i)$$
3. Intermediate work: List the vertices in order and repeat the first at the end for the shoelace sum.
Compute the cross terms $x_i y_{i+1}-x_{i+1} y_i$ for each edge.
For $D\to E$: $5\cdot 2-8\cdot 4=-22$.
For $E\to F$: $8\cdot(-2)-7\cdot 2=-30$.
For $F\to G$: $7\cdot(-4)-4\cdot(-2)=-20$.
For $G\to D$: $4\cdot 4-5\cdot(-4)=36$.
Sum of the cross terms $\sum k_i=-36$.
Signed area $A_{signed}=\tfrac12(-36)=-18$.
Area $A=|A_{signed}|=18$.
4. Centroid calculation: compute numerators then divide by $6A_{signed}$.
Compute each term for the $C_x$ numerator.
$(5+8)(-22)=-286$.
$(8+7)(-30)=-450$.
$(7+4)(-20)=-220$.
$(4+5)(36)=324$.
Sum for $C_x$ numerator $=-632$.
Using the centroid formula and $A_{signed}=-18$ gives $C_x=\frac{-632}{6\cdot(-18)}=\frac{632}{108}=\frac{158}{27}\approx 5.85185$.
Compute each term for the $C_y$ numerator.
$(4+2)(-22)=-132$.
$(2+(-2))(-30)=0$.
$(-2+ -4)(-20)=120$.
$(-4+4)(36)=0$.
Sum for $C_y$ numerator $=-12$.
Using the centroid formula gives $C_y=\frac{-12}{6\cdot(-18)}=\frac{12}{108}=\frac{1}{9}\approx 0.11111$.
Final answer: Area $A=18$ and centroid $C=\left(\frac{158}{27},\frac{1}{9}\right)\approx(5.85185,0.11111)$.
Polygon Centroid
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