[নোটঃ এই আর্টিকেলটি (1) Geometry বিভাগের অধীনে (1) Lines and Angels চ্যাপ্টারের অন্তর্গত, যা 〈1.1.a〉চিহ্ন দিয়ে প্রকাশ করা হয়েছে]

## 〈1.1〉(a) কোণ সম্পর্কিত প্রাথমিক আলোচনা

##### Angles

Two lines (রেখা) or line-segments (রেখাংশ) are required to form an angle. An angle is represented by \angle   sign .\angle ABC is an angle below.

##### Circle
• A circle has 360° angles at its center.

• When two lines intersect, four angles are formed, and the sum of the measures of the angles is 360°. Similarly when more than two lines intersect, the sum of all the angles is also 360°.

• All the angles formed around a point are also 360°.
##### Right angle(সমকোণ)

It is a 90° angle. When two lines are perpendicular, a right angle is formed, and it is expressed with sign. Below is a right angle.

##### Acute angles (সূক্ষ্ কোণ)

It smaller than 90°. On the other hand, angles larger than 90° are called Obtuse angles (স্থূলকোণ)

Here   \angle ABC is acute but \angle CBD is obtuse.

##### Straight angle (সরলকোণ)

It is a 180°. This is also a straight line, on which a point exists. \angle ABC is a straight line in the figure below.

##### Reflex angle (প্রবৃদ্ধ কোণ)

It is larger than 90 but smaller than 180.

Angles having common vertex and common leg. \angle x and  \angle y are adjacent angles in the figure below.

##### Complementary angles (পূরক কোণ)

These are two adjacent angles whose sum is 90°. p and q are complementary in the figure below, because their sum is 90°. 20° is the complementary angle for 70°.

##### Supplementary angles (সম্পূরক কোণ)

These are two adjacent angles whose sum is 180°. s and t are complementary in the figure below, because their sum is 180°. 120° is the supplementary angle for 60°.

##### Vertical angles (বিপ্রতীপ কোণ)

are formed when two lines intersect. Of the four angles formed, each opposite two are vertical. Vertical angles are equal. In the figure below a and b are vertical angles.

##### Corresponding Angles (অনুরূপ কোণ)

Are equal angles, formed on the same side when a non-perpendicular line intersects two parallel lines. In the figure below, line l and m are parallel and line k intersects them. The angles x and y are corresponding angle and x = y

##### Alternate angles (একান্তর কোণ)

These are the equal angles formed on the both side of a non-perpendicular line that intersects a pair of parallel lines. In the figure below, l and m are parallel lines having a non-perpendicular transversal k. the angles x and y formed are called alternate angles.

##### Straight line (সরল রেখা)

Straight line বা সরল রেখা যে 180° কোন বিন্দুতে  কোণ উৎপন্ন করে। নীচের ছবিতে দেখুনঃ

##### Parallel lines (সমান্তরাল রেখা)

এ ধরণের রেখাসমূহ পরষ্পর সমান্তরাল, কখনো পরষ্পরকে ছেদ করে না।