Example 1: Find the GCD of 18 and 24.
Solution: The GCD of 18 and 24 is 6.
Example 2: Find the LCM of 15 and 20.
Solution: The LCM of 15 and 20 is 60.
Example 1: Is 432 divisible by 4?
Solution: Yes, because the last two digits (32) are divisible by 4.
Example 2: Is 275 divisible by 11?
Solution: No, because the alternating sum of the digits 2−7+5=0 is not divisible by 11.
Diophantine equations are equations where integer solutions are required.
Example 1: Solve the Diophantine equation 3x+4y=17.
Solution: One solution is x=3, y=2.
Example 2: Solve 5x+7y=1.
Solution: One solution is x=−2, y=1.
These are problems requiring integer solutions to linear equations.
Example 1: Solve 6x+9y=15.
Solution: One solution is x=0, y=1.
Example 2: Solve 4x+5y=13.
Solution: One solution is x=3, y=−1.
Problem solving in number theory often involves applying theorems and methods such as GCD, LCM, and congruence.
Example 1: Solve 5x+7y=37 for integer values of x and y.
Solution: One solution is x=4, y=1.
Example 2: Find the remainder when 135 is divided by 11.
Solution: 135÷11=12, so the remainder is 3.
Two triangles are congruent if they have the same shape and size, which means their corresponding sides and angles are equal.
Example 1: Two triangles have sides of lengths 3 cm, 4 cm, and 5 cm. Are they congruent?
Solution: Yes, the triangles are congruent by the SSS (Side-Side-Side) rule.
Example 2: Two triangles have two equal angles and the included side is equal. Are they congruent?
Solution: Yes, the triangles are congruent by the ASA (Angle-Side-Angle) rule.
The triangle inequality theorem states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side.
Example 1: Can a triangle have sides of lengths 6 cm, 8 cm, and 15 cm?
Solution: No, because 6+8=14, which is not greater than 15. Therefore, these sides do not form a triangle.
Example 2: Check if sides of lengths 5 cm, 12 cm, and 13 cm can form a triangle.
Solution: Yes, 5+12>13, 5+13>12, and 12+13>5, so they form a triangle.
The Midpoint Theorem (MPT) states that the line segment joining the midpoints of two sides of a triangle is parallel to the third side and half its length. The Basic Proportionality Theorem (BPT), also known as Thales' Theorem, states that if a line is drawn parallel to one side of a triangle, it divides the other two sides proportionally.
Example 1: In a triangle, the midpoint of one side is joined to the midpoint of another side. What is the relationship between this line and the third side?
Solution: The line is parallel to the third side, and its length is half the length of the third side (MPT).
Example 2: In a triangle, a line is drawn parallel to one side, dividing the other two sides. What is the relationship between the divided sides?
Solution: The line divides the two sides proportionally (BPT).
Two triangles are said to be similar if their corresponding angles are equal and their corresponding sides are proportional.
Example 1: Are two triangles with angles 30o, 600, and 900 similar?
Solution: Yes, by the AA (Angle-Angle) similarity criterion.
Example 2: Two triangles have sides of lengths 5 cm, 7 cm, and 9 cm, and 10 cm, 14 cm, and 18 cm, respectively. Are they similar?
Solution: Yes, the triangles are similar by the SSS (Side-Side-Side) similarity rule, as the sides are proportional.
Important points in a triangle include the centroid, circumcentre, incenter, and orthocentre.
Example 1: What is the centroid of a triangle?
Solution: The centroid is the point where the medians of the triangle intersect. It divides each median into two segments, one of which is twice the length of the other.
Example 2: Where is the circumcentre of a right triangle located?
Solution: The circumcentre is at the midpoint of the hypotenuse.