[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।