4.1.2 Geometry⭐几何学

The two lines on the left intersect each other at a right angle, which is 90 degrees.

The two lines on the right are parallel, which means they never cross.

The adjacent sides of a rectangle are perpendicular, which means they meet at a 90 degree angle.

The opposite sides of a rectangle are equal and parallel, which means they never cross.

The area of a rectangle is the length times the width.

A circle is a 2 dimensional figure with a radius and circumference, which is the distance around the circle.

The area of a circle is pi r squared, where r is the radius and pi is the ratio of a circle's circumference to its diameter.

A sphere is a 3 dimensional figure with a radius and a volume, which is 4/3 pi r cubed.

All points on the surface of a sphere are equidistant from its center.

A right triangle is a 2 dimensional figure with 3 sides, two of which are perpendicular.

To find its area, multiply the length of the two perpendicular sides, a and b, and divide by two, A=1/2 ab (A equals one half ab).