Simple Geometric Mathematics
CIRCLE
Area of a Circle
The area of a circle is p ? r2 or p ? r ? r, where r is the length of its radius.
p is a number that is approximately 3.14159.
Radius: r
Diameter: d
Circumference: C
Area: K
d = 2r
C = 2 p r = p d
K = p r2 = p d2/4
C = 2 p r
K = p d2 /4
Area on the sphere
The area of a sphere of radius R is 4 R .
A great circle divides the sphere into two congruent hemispheres. Each of these will have area 2 R .
Another great circle, which meets the first at right angles, divides the sphere into four congruent lunes, each with area R .
Triangle
Consider a triangle with base length b and height h.
The area of the triangle is 1/2 ? b ? h.
Parallelogram
The area of a parallelogram is b ? h, where b is the length of the base of the parallelogram, and h is the corresponding height. To picture this, consider the parallelogram below:
Trapezoid
If a and b are the lengths of the two parallel bases of a trapezoid, and h is its height, the area of the trapezoid is
1/2 ? h ? (a + b) .
Cylinders
If r is the radius of the base and h is the altitude,
the area is 2 rh,
and the volume is r h.
Conicals
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