Let $ P,\ Q,\ R$ be the points such that $ BP: PC=CQ: QA=AR: RB=t: 1-t\ (0<t<1)$ for a triangle $ ABC$.
Denote $ K$ by the area of the triangle with segments $ AP,\ BQ,\ CR$ as side lengths and $ L$ by triangle $ ABC$, find $ \frac{K}{L}$ in terms of $ t$.
To find the ratio \(\frac{K}{L}\) where \(K\) is the area of the triangle formed by segments \(AP\), \(BQ\), and \(CR\), and \(L\) is the area of triangle \(ABC\), we will use vector analysis and properties of similar triangles.

1. **Define Vectors:**
   Let \( \vec{A} \), \( \vec{B} \), and \( \vec{C} \) be the position vectors of points \(A\), \(B\), and \(C\) respectively. Define:
   \[
   \vec{A} \rightarrow \vec{B} = \vec{z}, \quad \vec{B} \rightarrow \vec{C} = \vec{x}, \quad \vec{C} \rightarrow \vec{A} = \vec{y}
   \]
   Note that:
   \[
   \vec{x} + \vec{y} + \vec{z} = 0
   \]

2. **Express Points \(P\), \(Q\), and \(R\) in Terms of Vectors:**
   Given the ratios \(BP:PC = CQ:QA = AR:RB = t:1-t\), we can write:
   \[
   \vec{P} = (1-t)\vec{B} + t\vec{C} = (1-t)\vec{z} + t\vec{x}
   \]
   \[
   \vec{Q} = (1-t)\vec{C} + t\vec{A} = (1-t)\vec{x} + t\vec{y}
   \]
   \[
   \vec{R} = (1-t)\vec{A} + t\vec{B} = (1-t)\vec{y} + t\vec{z}
   \]

3. **Translate Vectors to Form Triangle:**
   The triangle formed by \(AP\), \(BQ\), and \(CR\) can be translated such that:
   \[
   \vec{AP} = \vec{P} - \vec{A} = (1-t)\vec{z} + t\vec{x} - \vec{A}
   \]
   \[
   \vec{BQ} = \vec{Q} - \vec{B} = (1-t)\vec{x} + t\vec{y} - \vec{B}
   \]
   \[
   \vec{CR} = \vec{R} - \vec{C} = (1-t)\vec{y} + t\vec{z} - \vec{C}
   \]

4. **Area Calculation:**
   The area of the triangle formed by \(AP\), \(BQ\), and \(CR\) can be found by considering the determinant of the matrix formed by these vectors. However, a simpler approach is to use the fact that the area of a triangle is proportional to the product of the lengths of its sides and the sine of the included angle.

5. **Proportionality and Similarity:**
   Since the triangles are similar by construction, the ratio of their areas is the square of the ratio of their corresponding sides. The sides \(AP\), \(BQ\), and \(CR\) are proportional to the sides of \(ABC\) by a factor of \(1-t+t^2\).

6. **Final Ratio:**
   Therefore, the ratio of the areas \( \frac{K}{L} \) is given by:
   \[
   \frac{K}{L} = (1-t+t^2)^2
   \]

The final answer is \(\boxed{(1-t+t^2)^2}\)