Question

why we cannot construct a triangle of given sides as 5cm,5cm and 10cm

Answer 1

because the sum of first two sides is not greater than the third side

Answer 2

according to the rule, sum of two sides of the triangle is greater than the third side.
5+5 is not greater than 10. so the triangle with the given sides is not possible.
also difference of two sides is smaller than the third side, but 10-5 is not smaller than 5. so the triangle isn't possible.