Here are four squares. The area of the little square=5.
What's the area of the blue triangle?
1/ Note that both small squares have area =5. The medium square is the size of four small squares so it has area=20. Amazingly, it doesn't matter how big the biggest square is!
2/ The green lines are parallel. Therefore, as the blue and pink triangles have the same base and same height, their areas are equal.
3/ Again, the green lines are parallel, so the blue triangle has the same area as the pink triangle.
4/ The grey triangle is half the 20 square, so the area = 10. This is the same as the original blue triangle!