ABC is right-angled triangle at B. Given that AB = 9 cm, AC = 15 cm and D and E are the mid-points of the sides AB and AC respectively, calculate the area of ΔADE.
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Solution
By pythogoras theorem, AB2+BC2=AC2 ⇒BC2=AC2−AB2 ⇒BC2=152−92 ⇒BC2=225−81=144 ⇒BC=12cm
As D and E are mid-points of AB and AC, by mid-point theorem, DE∥BC and DE=12BC=6cm
Since, DE∥BC,wehaveΔADEcongruenttoΔABC Area(ΔADE)Area(ΔABC)=DE2BC2 ⇒Area(ΔADE)12×AB×BC=(6)2BC2 ⇒Area(ΔADE)12×9×12=36144 ⇒Area(ΔADE)=14×9×6 ⇒Area(ΔADE)=274=13.5cm2