Final answer:
The cost of planting green grass in the triangular park is 0.
Step-by-step explanation:
To find the cost of planting green grass, we need to determine the area of the triangular park first. The area of a triangle can be calculated using Heron's formula: A = √(s(s-a)(s-b)(s-c)), where s is the semi-perimeter of the triangle and a, b, and c are the lengths of its sides. In this case, the semi-perimeter s = (120+50+170)/2 = 170.
Substituting these values into the formula gives us A = √(170(170-120)(170-50)(170-170)) = √(170*50*20*0) = 0. Since the area of the park is 0, the cost of planting green grass would also be 0.