Basic Concepts of Geometry Workbook-1
Angles: When two straight lines meet at a point they form an angle. In the figure, the angle is represented as ∠AOB. OA and OB are the arms of ∠AOB. Point O is the vertex of ∠AOB. The amount of turning from one arm (OA) to other (OB) is called the measure of the angle (AOB).
Right angle: An angle whose measure is 90o is called a right angle. ∠ BOA is a right angle in the figure.
Acute angle: An angle whose measure is less then one right angle (i.e., less than 90o), is called an acute angle.
Obtuse angle: An angle whose measure is more than one right angle i. e. 90o and less than two right angles
(i.e., less than 180o and more than 90o) is called an obtuse angle.
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Basic Concepts of Geometry Workbook-2
A triangle is a three sided closed figure
A triangle whose all the sides are unequal is called a scalene triangle. All the three angles will also be different in this case.
AB ≠ BC ≠ AC
A triangle, two of whose sides are equal in length is called an isosceles triangle. In this case two angles will be of equal measure and one will be different.
AB = AC ≠ BC
∠B = ∠ C □ ≠ ∠A
A triangle, all of whose sides are equal is called an equilateral triangle. All the three angles will be equal in this case and each of them will be of 60 degree measure.
AB = BC = AC
∠A = ∠B = ∠C = 60°
A triangle, each of whose angles is less than 90°, is called an acute angled triangle.
Acute Angle triangle
