In $\triangle ABC$, consider points $D$ and $E$ on $AC$ and $AB$, respectively, and assume that they do not coincide with any of the vertices $A$, $B$, $C$. If the segments $BD$ and $CE$ intersect at $F$, consider areas $w$, $x$, $y$, $z$ of the quadrilateral $AEFD$ and the triangles $BEF$, $BFC$, $CDF$, respectively. 

[list=a]
[*] Prove that $y^2 > xz$.


[*] Determine $w$ in terms of $x$, $y$, $z$.
[/list]

[asy]
import graph; size(10cm); 
real labelscalefactor = 0.5; /* changes label-to-point distance */
pen dps = linewidth(0.7) + fontsize(12); defaultpen(dps); /* default pen style */ 
pen dotstyle = black; /* point style */ 
real xmin = -2.8465032978885407, xmax = 9.445649196374966, ymin = -1.7618066305534972, ymax = 4.389732795464592;  /* image dimensions */


draw((3.8295013012181283,2.816337276198864)--(-0.7368327629589799,-0.5920813291311117)--(5.672613975760373,-0.636902634996282)--cycle, linewidth(0.5)); 
 /* draw figures */
draw((3.8295013012181283,2.816337276198864)--(-0.7368327629589799,-0.5920813291311117), linewidth(0.5)); 
draw((-0.7368327629589799,-0.5920813291311117)--(5.672613975760373,-0.636902634996282), linewidth(0.5)); 
draw((5.672613975760373,-0.636902634996282)--(3.8295013012181283,2.816337276198864), linewidth(0.5)); 
draw((-0.7368327629589799,-0.5920813291311117)--(4.569287648059735,1.430279997142299), linewidth(0.5)); 
draw((5.672613975760373,-0.636902634996282)--(1.8844000180622977,1.3644681598392678), linewidth(0.5)); 
label("$y$",(2.74779188172294,0.23771684184669772),SE*labelscalefactor); 
label("$w$",(3.2941097703568736,1.8657441499758196),SE*labelscalefactor); 
label("$x$",(1.6660824622277512,1.0025618859342047),SE*labelscalefactor); 
label("$z$",(4.288408327670633,0.8168138037986672),SE*labelscalefactor); 
 /* dots and labels */
dot((3.8295013012181283,2.816337276198864),dotstyle); 
label("$A$", (3.8732067323088435,2.925600853925651), NE * labelscalefactor); 
dot((-0.7368327629589799,-0.5920813291311117),dotstyle); 
label("$B$", (-1.1,-0.7565817154670613), NE * labelscalefactor); 
dot((5.672613975760373,-0.636902634996282),dotstyle); 
label("$C$", (5.763466626982254,-0.7784344310124186), NE * labelscalefactor); 
dot((4.569287648059735,1.430279997142299),dotstyle); 
label("$D$", (4.692683565259744,1.5051743434774234), NE * labelscalefactor); 
dot((1.8844000180622977,1.3644681598392678),dotstyle); 
label("$E$", (1.775346039954538,1.4942479857047448), NE * labelscalefactor); 
dot((2.937230516274804,0.8082418657164665),linewidth(4.pt) + dotstyle); 
label("$F$", (2.889834532767763,0.954), NE * labelscalefactor); 
clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle); 
[/asy]
### Part (a): Prove that \( y^2 > xz \)

1. **Apply the Lotus Ratio Lemma:**
   In a triangle \( \triangle ABC \), if lines \( BD \) and \( CE \) intersect at \( F \), then by the Lotus Ratio Lemma, we have:
   \[
   \frac{BF}{FD} = \frac{BE}{AE} \left( 1 + \frac{AD}{DC} \right)
   \]

2. **Assume for contradiction:**
   Assume \( y^2 < xz \). This implies:
   \[
   \frac{y}{x} < \frac{z}{y}
   \]

3. **Relate the areas to segment ratios:**
   Using the area ratios, we have:
   \[
   \frac{y}{x} = \frac{FC}{EF} \quad \text{and} \quad \frac{z}{y} = \frac{FD}{BF}
   \]

4. **Apply the Lotus Ratio Lemma:**
   Using the Lotus Ratio Lemma, we get:
   \[
   \frac{CD}{AD} \left( 1 + \frac{AE}{BE} \right) < \frac{AE}{BE} \left( \frac{DC}{AD + DC} \right)
   \]

5. **Simplify the inequality:**
   This simplifies to:
   \[
   AB \cdot AC < AE \cdot AD
   \]
   which is a contradiction because \( AB \cdot AC \) should be greater than or equal to \( AE \cdot AD \).

6. **Conclusion:**
   Therefore, our assumption \( y^2 < xz \) must be false, and hence:
   \[
   y^2 > xz
   \]

\(\blacksquare\)

### Part (b): Determine \( w \) in terms of \( x \), \( y \), and \( z \)

1. **Apply the Lotus Ratio Lemma again:**
   Using the Lotus Ratio Lemma, we have:
   \[
   \frac{BF}{FD} = \frac{BE}{AE} \left( 1 + \frac{AD}{DC} \right)
   \]

2. **Relate the areas to segment ratios:**
   Using the area ratios, we have:
   \[
   \frac{y}{z} = \frac{x + y}{w + z} \left( 1 + \frac{w + x}{y + z} \right)
   \]

3. **Simplify the equation:**
   This gives:
   \[
   \frac{x + y}{w + z} = \frac{y^2 + zy}{zx + zy}
   \]

4. **Solve for \( w \):**
   Rearranging the equation, we get:
   \[
   w + z = \frac{z(x + y)^2}{y^2 - zx}
   \]
   Therefore:
   \[
   w = \frac{z(x + y)^2}{y^2 - zx} - z
   \]
   Simplifying further:
   \[
   w = z \left( \frac{(x + y)^2}{y^2 - zx} - 1 \right)
   \]
   \[
   w = z \left( \frac{x^2 + 2xy + y^2}{y^2 - zx} \right)
   \]

\(\blacksquare\)

The final answer is \( \boxed{ w = z \left( \frac{x^2 + 2xy + y^2}{y^2 - zx} \right) } \)