Examples in real life
Setsquare, View from a ship to light house; Half sandwich; Diameter of a circle subtending 90° on the circumference; Ladder against a wall.
A triangle in which one of the angle measures 90°, is called a right-angled triangle or simply a right triangle. In a right-angled triangle, the side opposite to the right angle is called its hypotenuse and the other two sides are called 'legs'. From the angle sum theorem, it is obvious that a triangle can not have more than one right angle.
In Δ ABC, AC is the hypotenuse and in Δ PQR, PR is the hypotenuse. It is a common practice to represent a right angle as shown in Δ ABC or in Δ PQR without actually mentioning as 90°.
You will learn later the relationship between the hypotenuse and the legs of the right-angled triangle given by Pythagoras theorem.
Ex: What are the angles of a right isosceles triangle?
Sol: One of the angles is 90°. Being isosceles, two sides are equal and hence the base angles are also equal. Their sum in this case is 90°. Therefore, each base angle is equal to 45°. So, in any right isosceles triangle the angles are always 90, 45 and 45°. This is shown in Δ PQR above.
You will also learn later the definitions of six trigonometrical ratios and their relationships in a right-angled triangle.