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Basic Geometry

Basic Geometry

The surface area of a solid is the area of each surface added together.

There are few formulas to memorize (w00t!). The keys to success: make sure that you don't forget a surface and that you have the correct measurements.

Surface area is often used in construction. If you need to paint any 3‐D object you need to know how much paint to buy.

Surface Area of a Rectangular Prism

8 x 4 x 6

If we "unfold" the box, we get something that is called – in the geometry world – a "net".

8 x 4 x 6 net

Using the net we can see that there are six rectangular surfaces.


Side 14 x 832 cm2
Side 28 x 648 cm2
Side 34 x 832 cm2
Side 48 x 648 cm2
Side 54 x 624 cm2
Side 64 x 624 cm2
TOTAL208 cm2

If we study the table we will see that there are two of each surface. That's because the top and bottom of a rectangular prism are congruent, as are the two sides, and the front and back.

Surface Area of a Triangular Prism

Triangular Prism

If we break down our triangular prism into a net, it looks like this:

Triangular prism net

In a triangular prism there are five sides, two triangles and three rectangles.


Side 1½(9 × 4)18 cm2
Side 2½(9 × 4)18 cm2
Side 34.5 x 8.136.45 cm2
Side 49 x 8.172.9 cm2
Side 57.2 x 8.158.32 cm2
TOTAL203.67 cm2

Surface Area of a Cylinder
= 2(area of the circular base) + h(circumference)
= 2πr2 + 2πrh

Imagine a can of soup.

4.4 x 7.2 cylinder

If we use a can opener and cut off the top and bottom, and unroll the middle section, we would get:

cylinder net

Now you can see that we have two congruent circles, each with a radius of 4.4 cm and a rectangle with a width of 7.2 cm. The only measurement we are missing is the length. Remember when we unrolled the center section. Well, its length was wrapped around the circles, so it's the perimeter of the circle, i.e., the circumference. Therefore, we must find the circumference of a circle with radius 4.4 cm.

Circumference of a circle = dπ = (4.4 × 2)π = 8.8π ≈ 27.63 cm

cylinder net with circumference

Now, we can solve for surface area:


Circle 14.42 × π≈ 60.79 cm2
Circle 24.42 × π≈ 60.79 cm2
Center27.63 x 7.2≈ 198.94 cm2
TOTAL≈ 320.52 cm2

Surface Area of a Sphere = 4πr2

That great mathematician Archimedes, the one who gave us the formula for the volume of a sphere, spent many hours plugging away by candlelight to bring you this: the surface area of a sphere is 4 times the area of the center circle.

Surface Area of a Cone = πr2 + πrs

To find the surface area of a cone we need to find the area of the circular base and the area of the curved section. This one involves a new measurement, s, which is the length of the slanted part.

If you take apart the cone, you get two surfaces, the circular base and the curved sides. The area of the base is just πr2, and the area of the curved section is πrs.

Look Out: surface area is only two-dimensional and is expressed as units squared, not units cubed. This is because we are only dealing with the flat surfaces, not the inside space.

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