Ok, so
First, remember that:
We can use the following method to calculate the type of triangle:
1. Make c the length of the longest side, and calculate c².
2. Make a and b the length of the two shorter sides, and calculate a² + b².
3. Compare c² and a² + b².
4. Use the following conditions below to work out the type of triangle:
- If c² = a² + b² , that's a right-angled triangle.
- If c² > a² + b², that's an obtuse-angled triangle.
- If c² < a² + b², that's an acute-angled triangle.
If we replace our values:
c = 20cm. c² = 400cm²
a = 12cm
b = 15cm.
a² + b² = 144cm² + 225cm²
a² + b² = 369
As you can see, c² > a² + b², so, this is an obtuse-angled triangle.