Question
Let n > 2 be an integer. Suppose that there are n Metro stations in a city located along a circular path. Each pair of stations is connected by a straight track only. Further, each pair of nearest stations is connected by blue line, whereas all remaining pairs of stations are connected by red line. If the number of red lines is 99 times the number of blue lines, then the value of n is :
Options
Solution
Key Concepts and Formulas
- Combinations: The number of ways to choose items from a set of distinct items, where order doesn't matter, is given by . In this case, we use to find the total number of lines connecting pairs of stations.
- Circular Permutations: In a circular arrangement of objects, each object has two immediate neighbors. Therefore, there are connections between nearest neighbors.
- Problem Translation: Converting the word problem's conditions into a mathematical equation is crucial. In this case, the number of red lines is 99 times the number of blue lines.
Step-by-Step Solution
Step 1: Calculate the Total Number of Possible Lines
- What and Why: We need to find the total number of lines that can be drawn between any two stations. Since each pair of stations is connected by a line, we use combinations to choose 2 stations out of . Order doesn't matter (station A to B is the same line as B to A).
- Calculation: Let be the total number of lines.
Step 2: Determine the Number of Blue Lines
- What and Why: Blue lines connect nearest neighbor stations. In a circular arrangement, each station has two neighbors. Therefore, there are such connections.
- Calculation: Let be the number of blue lines.
Step 3: Determine the Number of Red Lines
- What and Why: Red lines connect all non-nearest neighbor stations. To find the number of red lines, we subtract the number of blue lines from the total number of lines.
- Calculation: Let be the number of red lines. Simplifying the expression:
Step 4: Formulate the Equation from the Given Condition
- What and Why: The problem states that the number of red lines is 99 times the number of blue lines. We translate this into an equation using the expressions we derived for and .
- Equation: Substituting the expressions:
Step 5: Solve the Algebraic Equation for
- What and Why: Now we solve the equation for . Since , we know , so we can safely divide both sides by .
- Solving: Divide both sides by (since ): Multiply both sides by 2: Add 3 to both sides:
Common Mistakes & Tips
- Confusing Total Lines with Red Lines: Remember to subtract the blue lines from the total lines to get the number of red lines.
- Circular vs. Linear Arrangements: Understand that in a circular arrangement, each element has two neighbors, leading to connections.
- Algebraic Simplification: Be careful with algebraic manipulations to avoid errors.
Summary
We calculated the total number of lines, the number of blue lines, and the number of red lines in terms of . Using the given condition that the number of red lines is 99 times the number of blue lines, we set up an equation and solved for , finding that .
Final Answer
The final answer is , which corresponds to option (A).