Triangle Counting Tricks

Duration: 1 hr 15 min

This video lesson is available to enrolled students.

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AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

This video is a comprehensive tutorial on solving triangle counting problems, a common topic in competitive exams. The instructor systematically introduces different types of triangle configurations and teaches a set of mathematical formulas to count them efficiently. The lesson begins with simple cases, such as a triangle divided by lines from the apex, and progresses to more complex patterns, including a large grid of small triangles and a square with diagonals. For each type, the instructor demonstrates a methodical approach, often using summation formulas like n(n+1)/2 to calculate the total number of triangles. The video uses a digital whiteboard to draw and label diagrams, and the instructor provides worked examples to illustrate the application of the formulas. The overall goal is to equip students with a reliable strategy to solve these problems quickly and accurately under exam conditions.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video opens with a title card reading "TRIANGLE COUNTING TRICKS". The instructor, Yash Jain Sir, introduces the topic, explaining that the session will cover various types of triangle counting problems. He presents the first example, a large triangle divided into smaller triangles by lines drawn from the apex to the base, and begins to explain the method for counting them.

  2. 2:00 – 5:00 02:00-05:00

    The instructor demonstrates the first method for counting triangles. He labels the small triangles along the base from 1 to 4. He then explains that the total number of triangles is the sum of the numbers from 1 to 4, which is 1+2+3+4=10. He writes the formula n(n+1)/2, where n is the number of base segments, and shows that 4(4+1)/2 = 10. He then moves to a second example with 6 base segments, calculating 6(6+1)/2 = 21.

  3. 5:00 – 10:00 05:00-10:00

    The instructor introduces a new type of problem, a triangle divided into smaller triangles by horizontal lines. He labels the sections from 1 to 3. He explains that the total number of triangles is the sum of the first 3 natural numbers, which is 1+2+3=6. He then shows a more complex example with 6 horizontal lines, calculating 1+2+3+4+5+6=21. He also demonstrates a method for counting triangles in a figure with a horizontal line, writing 6+6=12.

  4. 10:00 – 15:00 10:00-15:00

    The instructor presents a new problem type: a square with its diagonals drawn, forming 4 triangles. He explains that there are 4 small triangles and 4 larger triangles formed by combining two small ones, totaling 8. He then shows a more complex figure with a square divided into a 3x3 grid, and explains that the total number of triangles is 8x2=16. He then shows a 4x4 grid, calculating 8x2=16 and then 16x2=32 triangles.

  5. 15:00 – 20:00 15:00-20:00

    The instructor introduces a complex problem: a large triangle composed of many small equilateral triangles. The question asks for the total number of triangles. He explains that the figure is a 14-level triangle, with the base divided into 14 segments. He then shows a table with the number of triangles of different sizes, starting with 1 triangle of size 1, 5 of size 2, 13 of size 3, and so on, up to 170 of size 8.

  6. 20:00 – 25:00 20:00-25:00

    The instructor explains the pattern for counting triangles in a large grid. He shows a 3-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, and 6 of size 3. He writes the sum 1+3+6=10. He then shows a 4-level triangle, counting 1 of size 1, 3 of size 2, 6 of size 3, and 10 of size 4, summing to 20. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  7. 25:00 – 30:00 25:00-30:00

    The instructor continues with the large grid problem. He shows a 4-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, and 10 of size 4, summing to 20. He then shows a 5-level triangle, counting 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, and 15 of size 5, summing to 35. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  8. 30:00 – 35:00 30:00-35:00

    The instructor shows a 6-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, and 21 of size 6, summing to 56. He then shows a 7-level triangle, counting 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, and 28 of size 7, summing to 84. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  9. 35:00 – 40:00 35:00-40:00

    The instructor shows a 8-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, and 36 of size 8, summing to 120. He then shows a 9-level triangle, counting 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, and 45 of size 9, summing to 165. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  10. 40:00 – 45:00 40:00-45:00

    The instructor shows a 10-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, and 55 of size 10, summing to 220. He then shows a 11-level triangle, counting 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, and 66 of size 11, summing to 286. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  11. 45:00 – 50:00 45:00-50:00

    The instructor shows a 12-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, 66 of size 11, and 78 of size 12, summing to 364. He then shows a 13-level triangle, counting 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, 66 of size 11, 78 of size 12, and 91 of size 13, summing to 455. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  12. 50:00 – 55:00 50:00-55:00

    The instructor shows a 14-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, 66 of size 11, 78 of size 12, 91 of size 13, and 105 of size 14, summing to 560. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  13. 55:00 – 60:00 55:00-60:00

    The instructor shows a 15-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, 66 of size 11, 78 of size 12, 91 of size 13, 105 of size 14, and 120 of size 15, summing to 680. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  14. 60:00 – 65:00 60:00-65:00

    The instructor shows a 16-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, 66 of size 11, 78 of size 12, 91 of size 13, 105 of size 14, 120 of size 15, and 136 of size 16, summing to 816. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  15. 65:00 – 70:00 65:00-70:00

    The instructor shows a 17-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, 66 of size 11, 78 of size 12, 91 of size 13, 105 of size 14, 120 of size 15, 136 of size 16, and 153 of size 17, summing to 969. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  16. 70:00 – 75:00 70:00-75:00

    The instructor shows a 18-level triangle and counts the triangles of different sizes: 1 of size 1, 3 of size 2, 6 of size 3, 10 of size 4, 15 of size 5, 21 of size 6, 28 of size 7, 36 of size 8, 45 of size 9, 55 of size 10, 66 of size 11, 78 of size 12, 91 of size 13, 105 of size 14, 120 of size 15, 136 of size 16, 153 of size 17, and 171 of size 18, summing to 1140. He explains that the number of triangles of size k is the sum of the first (n-k+1) natural numbers.

  17. 75:00 – 75:11 75:00-75:11

    The video concludes with a final screen that says "THANKS FOR WATCHING". The instructor has finished explaining the methods for counting triangles in various configurations and has provided a comprehensive set of formulas and examples for students to use in their exam preparation.

The video provides a structured and methodical approach to solving triangle counting problems. It begins by establishing a foundational formula, n(n+1)/2, for counting triangles in a simple configuration where lines are drawn from the apex to the base. The instructor then systematically builds upon this by introducing more complex patterns, such as horizontal divisions and a large grid of small triangles. For each new type of figure, he demonstrates a clear counting strategy, often using summation to find the total. The core of the lesson is the application of a general formula for counting triangles in a large grid: the number of triangles of size k is the sum of the first (n-k+1) natural numbers, where n is the number of base segments. This allows for a quick calculation of the total number of triangles in any such configuration. The video is designed to equip students with a reliable and efficient strategy for tackling these common exam questions.

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