Solution: the first one: the third side = the square of 20 under the root MINUS the square of 15 = the square of 175 meters under the root.

20×15÷ 2 =150m2,

The second type: the third side = the square of 20 under the root plus the square of 15 = 625m under the root = 25m.

So the triangular green area =25× 12÷2= 150 square meters.

To sum up, the area of this triangular green space is 150 square meters.