Can we construct a ∆ABC in which AB=5cm, BC=4cm and AC=9cm? Why?
We know that,
The sum of two sides must be greater than the third side
In this case, we have
AB + BC = 5 + 4 = 9 cm
AC = 9 cm
∴ AC must be greater than the sum of AB and BC
Hence, the sum of two sides is not greater than the third side. So, 
cannot be constructed
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