If all the angles of a triangle is less than 90\\degree, it is called an acute-angled triangle. If any one of the angles of a triangle is greater than 90\\degree but less than 180\\degree, it is called an obtuse-angled triangle. If any one of the angles of a triangle is 90\\degree, it is called a right-angled triangle.

Think of the geometry of prism.

Think of the plane surfaces of a solid

Only sphere is a 3-D figure out of these

Interior angle formula for a polygon

Interior angle formula for a polygon

The L-shaped figure forms a right angle (90\\degree), angle less than 90\\degree is acute angle, angle measuring between 90\\degree and 180\\degree is obtuse angle, angle measuring exactly 180\\degree is straight angle, angle measuring greater than 180\\degree but less than 360\\degree is reflex angle, if there is no angle between the two lines then it is zero\\degree, if the angle is exactly 360\\degree then it is a complete angle.

The triangle with all the three sides as equal is known as equilateral triangle. The triangle with all the three sides as different is known a scalene triangle. The triangle with any two sides equal is known as isosceles triangle.

Interior angle formula for a polygon

Properties of Parallelogram