Question

Are the following triangles similar? Justify your answer.

Answer 1

Yes, the smaller triangle is multiplied by two on each side, you then end up with the larger triangle.
So, the left triangle is 1/2 the size of the right.

Answer 2

While radsurlak's way is quick and efficient, here is a way to confirm their similarity.

Try taking each set of corresponding sides and placing them into a fraction.
2.5 over 5
4.5 over 9
4 over 8

Simplify them and if they are all have the same ratio, then the two triangles are similar!