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Using Triangles to Find the Area of Trapezia

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Learning text on the topic Using Triangles to Find the Area of Trapezia

Finding the Area of Trapezia

Imagine you're an artist planning to paint a canvas that's shaped like a door (parallelogram) or a kite (trapezium). Before you start, you need to know how much paint you'll need. That's where calculating the area of a shape, or the space inside these shapes, becomes very important. Today, we're going to learn exactly how to do that, making sure we have enough paint and don't run out!

Parallelogram: A shape where opposite sides are parallel and equal in length, like a slanted rectangle.

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Trapezium: A shape with at least one pair of parallel sides. Think of it as a stretched-out triangle with a flat top. When we have more than one trapezium, we use the plural trapezia.

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Finding the Area of a Trapezium

The area is the amount of space inside a shape. For trapezia, there are a variety of different methods that can be used to find the area.

Method Description When to Use
Decomposing into Two Triangles This method involves dividing the trapezium into two triangles by drawing a line from one vertex to the opposite vertex. Then, calculate the area of each triangle and add them to find the total area. Useful when the height and the lengths of the parallel sides (bases) are known. It simplifies the calculation by breaking the shape into simpler parts.
Using a Formula The formula for the area of a trapezium is $\frac{base_1 + base_2}{2} \times height$. This method is straightforward and quick, requiring only the lengths of the bases and the height. Best for situations where quick calculations are needed, and all the necessary measurements are readily available.

Decomposing a Trapezium

Imagine cutting a trapezium into two smaller triangles to make it easier to measure. We can measure the area of each triangle and then add them together to find the total area of the trapezium. This is called decomposing a shape.

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Decomposition of a shape refers to the process of breaking down a complex geometric figure into simpler parts, such as triangles, rectangles, or other polygons, that are easier to analyse or measure. This technique is often used in mathematics to simplify the calculation of areas, volumes and other properties of shapes.

Find the area by decomposing it into two triangles and finding the sum of their area.

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The trapezium has two base measurements, $6$ cm and $10$ cm, and the height is $4$ cm. Decompose the figure into two triangles.

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Now each triangle uses one of the bases of $6$ cm and $10$ cm. Use the formula for the area of a triangle ($A=\frac{1}{2}bh$) to find the area of each triangle separately, and then find the sum of their areas.

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Calculation for Triangle A Calculation for Triangle B
Base $b = 6$ cm Base $b = 10$ cm
Height $h = 4$ cm Height $h = 4$ cm
Area $A = \frac{1}{2} \times b \times h$ Area $A = \frac{1}{2} \times b \times h$
$A = \frac{1}{2} \times 6 \times 4 = 12$ cm$^2$ $A = \frac{1}{2} \times 10 \times 4 = 20$ cm$^2$

Next, find the sum of the two triangles.

$12 \text{cm}^2 + 20 \text{cm}^2 = 32 \text{cm}^2$

The total area of the trapezium is $\bf{32 \text{cm}^2}$.

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The trapezium has two base measurements, $8$ cm and $12$ cm, and the height is $5$ cm. Decompose the figure into two triangles.

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The trapezium has two base measurements, $5$ cm and $15$ cm, and the height is $6$ cm. Decompose the figure into two triangles.

Using the Formula to Find the Area of a Trapezium

Finding the area of a trapezium can be done quickly using a formula. This method is efficient and requires just a few measurements: the lengths of the two parallel sides (bases) and the distance between them (height).

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Area of a Trapezium Formula

  • $base_1$ and $base_2$: These are the lengths of the two parallel sides of the trapezium. It doesn't matter which one you call $base_1$ or $base_2$; just make sure you use both.
  • Height: This is the perpendicular distance between the two bases. It's important not to confuse this with the slant height or the length of the non-parallel sides. 22477_tov-10.svg

Find the area of a trapezium with $base_1 = 8$ cm, $base_2 = 5$ cm, and a height of 4 cm.

  • Identify the bases and the height: $base_1 = 8$ cm, $base_2 = 5$ cm, and the height = 4 cm.

  • Substitute into the formula: $ \text{Area} = \frac{(8 + 5)}{2} \times 4 $

  • Solve: $ \text{Area} = \frac{13}{2} \times 4 = 6.5 \times 4 = 26 \, \text{cm}^2 $

The area of the trapezium is 26 cm².

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Find the area of a trapezium with bases of 6 cm and 10 cm, and a height of 3 cm.

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Calculate the area of a trapezium with bases of 7 cm and 12 cm, and a height of 5 cm.
Determine the area of a trapezium with bases of 9 cm and 15 cm, and a height of 6 cm.

Finding the Area of a Trapezium – Summary

Key Learnings from this Text:

  • Understanding the area of trapezia is crucial for various mathematical and real-life applications.
  • Like all problems, finding the area of a trapezium can be done with a variety of different methods, including decomposition, and using a formula.
  • Decomposing shapes and using formulas are effective methods to find areas.
  • Knowing how to calculate these areas helps in planning, designing and executing projects that involve these shapes.

Finding the Area of a Trapezium – Frequently Asked Questions

What is a parallelogram?
How do you find the area of a parallelogram?
What is a trapezium?
How can you find the area of a trapezium?
Why is it useful to decompose a trapezium into triangles to find its area?
Can the formula for the area of a trapezium be used for all trapezia?
What measurements do you need to find the area of a trapezium using the formula?
How do you find the height of a trapezium?
Why is understanding the area of shapes important?
How can knowing the area of parallelograms and trapezia help in real life?
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