Finding the Area of Squares and TrianglesDate: 06/20/99 at 22:06:00 From: Sky Subject: I just don't get area Hi - Could you please explain area to me? Sincerely, Sky Date: 06/21/99 at 18:19:16 From: Doctor Rick Subject: Re: I just don't get area Hi, Sky. You have asked a broad question. I don't know just what about area is giving you trouble. Let's just go back to the basics - thinking about the basics may help you see harder problems in a new light. We start thinking about area by considering squares. Suppose you have a tile floor whose tiles are 1 foot by 1 foot. (You might even want to find a tile floor, and at least pretend the tiles are 1 foot square.) We say that the area of each square is 1 square foot. Mark out a rectangle on the floor with masking tape. Make this first rectangle right along the lines. Then you can find the area of the rectangle just by counting the tiles: +----+----+----+----+ | 1 | 2 | 3 | 4 | | | | | | +----+----+----+----+ | 5 | 6 | 7 | 8 | | | | | | +----+----+----+----+ | 9 | 10 | 11 | 12 | | | | | | 5 feet +----+----+----+----+ | 13 | 14 | 15 | 16 | | | | | | +----+----+----+----+ | 17 | 18 | 19 | 20 | | | | | | +----+----+----+----+ 4 feet This rectangle, 4 feet by 5 feet, has an area of 20 square feet - it contains 20 tiles. And of course you realize that this is 4 x 5. You can still find the area of a rectangle by multiplying the length by the width even if the sides aren't an integer number of feet: +----+----+----+----+--+ | 1 | 2 | 3 | 4 | | | | | | | | +----+----+----+----+--+ | 5 | 6 | 7 | 8 | | | | | | | | +----+----+----+----+--+ | 9 | 10 | 11 | 12 | | | | | | | | 5.5 feet +----+----+----+----+--+ | 13 | 14 | 15 | 16 | | | | | | | | +----+----+----+----+--+ | 17 | 18 | 19 | 20 | | | | | | | | +----+----+----+----+--+ | | | | | | +----+----+----+----+--+ 4.5 feet The area is 5.5 x 4.5 = 24.75 square feet. You can still count: 20 whole tiles, 9 half tiles, and 1 quarter tile total 24 3/4 tiles. The interesting part comes when we look at triangles. You would think diagonal lines would mess everything up, but they don't if you see the trick. +----------------------+ |\ | | \ | | \ | | \ | | \ | | \ | | \ | | \ | 5.5 feet | \ | | \ | | \ | | \ | | \ | | \ | | \ | | \| +----------------------+ 4.5 feet These 2 triangles are the same size and shape -- you could cut one out and place it exactly on top of the other. They have the same area. Both triangles together have an area of 5.5 x 4.5 square feet. One triangle alone has half this area, or (5.5 x 4.5)/2 = 12.375 square feet. The formula for the area of a right triangle is Area = (width x height) / 2 What if the triangle is not a right triangle? We can do just the same thing, with a little more cutting. B +---------+-------------------------+ | /|\ | | 1 / | \ | | / | \ 2 | | / | \ | | / | \ | | / 1 | \ | | / | 2 \ | | / | \ | |/ | \| +---------+-------------------------+ A D C The area of triangle ABC is exactly half the area of the rectangle. How do I know? Make the cuts shown. The two triangles labeled "1" are the same size and shape, so they have the same area. The two triangles labeled "2" also have the same area. A "1" and a "2" make up the triangle. Two of each make up the rectangle. Therefore the rectangle has twice the area of the triangle. The formula is Area of ABC = 1/2 base x height where the base is the length of the bottom of the triangle (the length of the rectangle), and the height is the length of BD (the height of the rectangle). What is important to notice here is that BD is NOT on of the sides of the triangle. It is a line from the corner (vertex) that is not on the base, drawn down to the base, and PERPENDICULAR to it - just as the sides of the rectangle are perpendicular (at right angles) to the base. We could go on to other shapes: parallelograms and trapezoids. If you forget the formulas for these, you can re-invent them for yourself by thinking jigsaw puzzles. Look at these "puzzles" and see if you can come up with the area on your own - you don't need the formulas. .......+---------------+------+ : / | / : / | / : / 5 feet | / Hint: move the triangle on the right : / | / to the left side as shown. : / | / :/ |/ +----------------------+ 6 feet 4 feet +---------------+ /| |\ /|\ / | | \ / | \ / | | \ / | \ / |5 feet | \ / | \ / | | \ / |5 ft \ / | 4 feet | \ / | \ +------+---------------+-----------+ +------+-----------+ 9 feet ? Hint: Make the 2 side triangles into the one triangle as shown. What is the base of the big triangle? Now you have 2 areas to add together. I haven't talked about circles, but everything else you have learned about can be solved by cutting in up into rectangles and triangles. This idea is much more important than memorizing formulas. You can't be sure you remembered the right formula for the shape, but you can be sure whether you're looking at a triangle or a rectangle. I hope this helps you. If you have specific questions, please ask them - we're glad to help. - Doctor Rick, The Math Forum http://mathforum.org/dr.math/ |
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