Why the sum of two sides of a triangle musb ne greater tnan the length of the third side?

Because the Pythagorus Theory says so. if they werent, it couldn't be a triangle. (This is only if this is a right triangle i guess)

You have 2 sides that are 2 cms and you have 6 sides that are 1/2 cm and you have 4 sides that are 1 cm. the smallest sides are 1/2 the medium sized sides are 1 and the smallest are 1/2. so to find the perimeter you have to add them all up together so your answer will be 11 cm's