Area of a polygon
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Chapter 7: Measurement
 Use paper or cardboard for the net of solids to help learners see the different heights, particularly perpendicular and slanted heights.
 Units are compulsory when working with real life contexts.
 Sketches are valuable and important tools.
 Rounding off should only be done in the last step and level of accuracy should be relevant to the context.
This chapter is a revision of perimeters and areas of two dimensional objects and volumes of three dimensional objects. We also examine different combinations of geometric objects and calculate areas and volumes in a variety of reallife contexts.
7.1 Area of a polygon (EMBHV)
Square  \(\text{Area}={s}^{2}\)  
Rectangle  \(\text{Area}=b\times h\)  
Triangle  \(\text{Area}=\frac{1}{2}b\times h\) 
Trapezium  \(\text{Area}=\frac{1}{2}\left(a+b\right)\times h\)  
Parallelogram  \(\text{Area}=b\times h\)  
Circle  \(\text{Area}=\pi {r}^{2}\) \(\left(\text{Circumference}=2\pi r\right)\) 
Worked example 1: Finding the area of a polygon
\(ABCD\) is a parallelogram with \(DC = \text{15}\text{ cm}\), \(h = \text{8}\text{ cm}\) and \(BF = \text{9}\text{ cm}\).
Calculate:
 the area of \(ABCD\)
 the perimeter of \(ABCD\)
Determine the area
The area of a parallelogram \(ABCD =\) base \(\times\) height:
\begin{align*} \text{Area} &= \text{15} \times \text{8} \\ &= \text{120}\text{ cm$^{2}$} \end{align*}Determine the perimeter
The perimeter of a parallelogram \(ABCD = 2DC + 2BC\).
To find the length of \(BC\), we use \(AF \perp BC\) and the theorem of Pythagoras.
\begin{align*} \text{In } \triangle \text{ ABF:} \quad AF^2 &= AB^2  BF^2 \\ &= \text{15}^2  \text{9}^2 \\ &= \text{144} \\ \therefore AF &= \text{12}\text{ cm} \end{align*}\begin{align*} \text{Area} ABCD &= BC \ \times AF \\ \text{120} &= BC \times \text{12} \\ \therefore BC &= \text{10}\text{ cm} \end{align*}\begin{align*} \therefore \text{Perimeter} ABCD &=2(\text{15})+2(\text{10}) \\ &= \text{50}\text{ cm} \end{align*}Area of a polygon
Vuyo and Banele are having a competition to see who can build the best kite using balsa wood (a lightweight wood) and paper. Vuyo decides to make his kite with one diagonal \(\text{1}\) \(\text{m}\) long and the other diagonal \(\text{60}\) \(\text{cm}\) long. The intersection of the two diagonals cuts the longer diagonal in the ratio \(\text{1}:\text{3}\).
Banele also uses diagonals of length \(\text{60}\) \(\text{cm}\) and \(\text{1}\) \(\text{m}\), but he designs his kite to be rhombusshaped.
Same amount of paper is required for both designs. Vuyo's designs uses more balsa wood.
\(O\) is the centre of the bigger semicircle with a radius of \(\text{10}\) \(\text{units}\). Two smaller semicircles are inscribed into the bigger one, as shown on the diagram. Calculate the following (in terms of \(\pi\)):
Karen's engineering textbook is \(\text{30}\) \(\text{cm}\) long and \(\text{20}\) \(\text{cm}\) wide. She notices that the dimensions of her desk are in the same proportion as the dimensions of her textbook.
If the desk is \(\text{90}\text{ cm}\) wide, calculate the area of the top of the desk.
Karen uses some cardboard to cover each corner of her desk with an isosceles triangle, as shown in the diagram:
Calculate the new perimeter and area of the visible part of the top of her desk.
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