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[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 .

Adjacent angles (সন্নিহিত কোণ)

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।

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