Practice Questions - Part 1

Duration: 44 min

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This educational video presents a series of geometry problems, focusing on the properties of squares, polygons, and triangles. The lecture begins with a problem involving a square ABCD, where a new square EFGH is formed by joining the midpoints of its sides, and this process is repeated to form smaller squares KLMN and PQRS. The instructor analyzes the relationship between the side lengths and areas of these nested squares, demonstrating that the area of each subsequent square is half of the previous one, leading to a ratio of 1:8 for the areas of KLMN and ABCD. The video then transitions to a problem about a sequence of squares constructed from a 10 cm line segment, where each new square uses the diagonal of the previous one as its side. The instructor derives a geometric progression for the side lengths, showing that the length of the nth square is 10 / (√2)^n, and solves for the smallest n where this length is less than 1/100. The final segment covers polygon angle sums, using the formula (n-2) × 180° for a regular n-gon, and applies it to a decagon. It also addresses triangle inequality, determining the possible integer values for the third side of a triangle with sides 12 cm and 7 cm, and calculating the sum of all such values. The video concludes with a problem on obtuse-angled triangles, using the Pythagorean theorem to find the range of the third side and counting the number of integer solutions.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video opens with a title card for 'GEOMETRY & MENSURATION'. It then presents a problem on a blackboard: 'In the given figure, EFGH is formed by joining midpoints of consecutive sides of square ABCD. Figures PQRS and KLMN are also formed similarly. The ratio of areas of KLMN and ABCD is __'. The instructor, Yash Jain Sir, begins to explain the problem, referencing the diagram which shows a large square ABCD with smaller squares EFGH, PQRS, and KLMN nested inside, each rotated 45 degrees relative to the previous one. The on-screen text clearly states the question and the multiple-choice options (A) 1:4, (B) 1:8, (C) 1:16, (D) 1:4√2.

  2. 2:00 – 5:00 02:00-05:00

    The instructor begins to solve the first problem. He draws a square and explains that joining the midpoints of a square's sides creates a new, smaller square. He uses the Pythagorean theorem to find the side length of the new square. If the original side is 'a', the new side is √((a/2)² + (a/2)²) = √(a²/2) = a/√2. He then calculates the area of the new square as (a/√2)² = a²/2, which is half the area of the original square. He applies this logic to the next iteration, showing that the area of PQRS is half of EFGH, and the area of KLMN is half of PQRS. He concludes that the area of KLMN is 1/8 of the area of ABCD, making the ratio 1:8.

  3. 5:00 – 10:00 05:00-10:00

    The instructor continues to explain the area ratio. He writes the formula for the area of a square, A = s², and shows the progression of side lengths: s, s/√2, s/(√2)², s/(√2)³. He then calculates the area of the third square (KLMN) as (s/(√2)³)² = s²/(√2)⁶ = s²/8. He confirms that the ratio of the area of KLMN to ABCD is 1/8, which corresponds to option (B). He then transitions to the next problem, which involves a line segment of 10 cm.

  4. 10:00 – 15:00 10:00-15:00

    The video presents a new problem: 'A line 10 cm long is drawn. A square is constructed with this line segment as diagonal. Then a second square is drawn with a side of the first square as diagonal. This process is repeated 'n' times, until the length of the nth square is less than 1/100. What is the least value of 'n'?'. The instructor draws a square with a diagonal of 10 cm. He uses the Pythagorean theorem to find the side length of the first square: a² + a² = 10², so 2a² = 100, a² = 50, a = √50 = 5√2. He then draws a second square with this side as its diagonal, and so on, illustrating the geometric progression.

  5. 15:00 – 20:00 15:00-20:00

    The instructor derives the general formula for the side length of the nth square. He shows that the side length of the first square is 10/√2. The side length of the second square is (10/√2)/√2 = 10/(√2)². He establishes the pattern: the side length of the nth square is 10/(√2)^n. He sets up the inequality 10/(√2)^n < 1/100 and solves for n. He multiplies both sides by 100(√2)^n to get 1000 < (√2)^n. He then takes the logarithm of both sides to find n > log(1000)/log(√2) = 3/log(√2) = 3/(0.5*log(2)) = 6/log(2). He calculates that log(2) is approximately 0.3010, so n > 6/0.3010 ≈ 19.93. Therefore, the least integer value of n is 20.

  6. 20:00 – 25:00 20:00-25:00

    The video moves to a new problem: 'In a regular decagon, what is the sum of all interior angles?'. The instructor writes the formula for the sum of interior angles of a polygon: (n-2) × 180°. He identifies that a decagon has n=10 sides. He substitutes the value into the formula: (10-2) × 180° = 8 × 180° = 1440°. He then presents a related problem: 'A regular polygon with 'n' sides has interior angle measuring 178°. What is the value of 180/n?'. He writes the formula for the interior angle of a regular polygon: ((n-2) × 180°)/n = 178°. He solves the equation: 180n - 360 = 178n, which gives 2n = 360, so n = 180. Therefore, 180/n = 180/180 = 1.

  7. 25:00 – 30:00 25:00-30:00

    The next problem is: 'The two sides of a triangle are 12 cm and 7 cm. If the third side is an integer, find the sum of all the values of the third side?'. The instructor draws a triangle and applies the triangle inequality theorem. He writes the two inequalities: 12 + 7 > c (so c < 19) and |12 - 7| < c (so c > 5). Therefore, the third side c must be an integer such that 5 < c < 19. The possible integer values are 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18. He then calculates the sum of these values: 6+7+8+...+18. He uses the formula for the sum of an arithmetic series: (number of terms)/2 × (first term + last term). The number of terms is 18-6+1=13. The sum is 13/2 × (6+18) = 13/2 × 24 = 13 × 12 = 156.

  8. 30:00 – 35:00 30:00-35:00

    The video presents a problem on obtuse-angled triangles: 'Consider obtuse angled triangles with sides 8 cm, 15 cm and x cm. If x is an integer, then how many such triangles exist?'. The instructor first applies the triangle inequality: 8+15 > x (x < 23) and |15-8| < x (x > 7). So, 7 < x < 23. For the triangle to be obtuse, the square of the longest side must be greater than the sum of the squares of the other two sides. He considers two cases: if x is the longest side, then x² > 8² + 15² = 64 + 225 = 289, so x > √289 = 17. If 15 is the longest side, then 15² > 8² + x², so 225 > 64 + x², which gives x² < 161, so x < √161 ≈ 12.68. Therefore, x < 13.

  9. 35:00 – 40:00 35:00-40:00

    The instructor combines the conditions for the obtuse triangle. From the triangle inequality, 7 < x < 23. From the obtuse condition, either x > 17 or x < 13. He considers the two cases. Case 1: x > 17 and 7 < x < 23, so 17 < x < 23. The integer values are 18, 19, 20, 21, 22. Case 2: x < 13 and 7 < x < 23, so 7 < x < 13. The integer values are 8, 9, 10, 11, 12. He adds the number of values from both cases: 5 + 5 = 10. Therefore, there are 10 such triangles.

  10. 40:00 – 43:30 40:00-43:30

    The video concludes with a final 'THANKS FOR WATCHING' screen. The instructor has completed all the problems, which covered the properties of squares, the sum of interior angles of polygons, the triangle inequality, and the conditions for a triangle to be obtuse. The key concepts demonstrated were the use of geometric formulas, algebraic manipulation, and logical reasoning to solve problems involving geometric figures.

The video is a comprehensive lecture on geometry, systematically progressing through a series of problems that reinforce fundamental concepts. It begins with the properties of nested squares, demonstrating a geometric sequence in side lengths and areas, which is a powerful application of the Pythagorean theorem. The lesson then transitions to a problem involving a geometric progression of square side lengths, requiring the use of logarithms to solve for the number of iterations. This is followed by a review of the formula for the sum of interior angles of a polygon, applied to a decagon and a general regular polygon. The final segment focuses on triangle properties, using the triangle inequality to find possible side lengths and the Pythagorean theorem to determine the conditions for a triangle to be obtuse. The synthesis of these topics shows a clear pedagogical structure, moving from basic shapes to more complex applications of geometric principles, all grounded in algebraic problem-solving.