Question 88

Logical Reasoning Line Easy

Ten points are marked on a straight line and eleven points are marked on another straight line. How many triangles can be constructed with vertices from among the above points?

(A) 495
(B) 550
(C) 1045
(D) 2475
✓ Correct Answer: Option C Verified Solution

📘 Step-by-Step Solution & Detailed Explanation:

Logical Reasoning Solution

Solution Breakdown

To form a triangle, we need to select 3 points. The condition is that these 3 points cannot all be on the same straight line (they must be non-collinear). We have two lines with points:

  • Line 1 has 10 points.
  • Line 2 has 11 points.

A triangle can be formed in two distinct ways:

  1. By selecting 2 points from Line 1 and 1 point from Line 2.
  2. By selecting 2 points from Line 2 and 1 point from Line 1.

We will use the combination formula, $C(n, k) = \frac{n!}{k!(n-k)!}$, to calculate the number of ways for each case.


Case 1: 2 points from Line 1 (10 points) and 1 point from Line 2 (11 points)

  • Ways to choose 2 points from 10: $C(10, 2) = \frac{10 \times 9}{2 \times 1} = 45$
  • Ways to choose 1 point from 11: $C(11, 1) = 11$
  • Number of triangles for this case: $45 \times 11 = 495$

Case 2: 2 points from Line 2 (11 points) and 1 point from Line 1 (10 points)

  • Ways to choose 2 points from 11: $C(11, 2) = \frac{11 \times 10}{2 \times 1} = 55$
  • Ways to choose 1 point from 10: $C(10, 1) = 10$
  • Number of triangles for this case: $55 \times 10 = 550$

Total Number of Triangles

To find the total number of possible triangles, we add the results from both cases.

Total Triangles = (Triangles from Case 1) + (Triangles from Case 2)

Total Triangles = $495 + 550 = \mathbf{1045}$

This corresponds to option C.

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