Isosceles trapezium

Calculate the area of an isosceles trapezium ABCD if a = 10cm, b = 5cm, c = 4cm.

Correct answer:

S =  28 cm²

Step-by-step explanation:

$$\begin{gathered} a=10\text{ cm } \\ b=5\text{ cm } \\ c=4\text{ cm } \\ x=(a-c)\mathrm{/}2=(10-4)\mathrm{/}2=3\text{ cm } \\ h=\sqrt{b^2-x^2}=\sqrt{5^2-3^2}=4\text{ cm } \\ S={\displaystyle \frac{a+c}{2}}\cdot h={\displaystyle \frac{10+4}{2}}\cdot 4=28{\text{cm}}^2 \end{gathered}$$

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