Answer:
Step-by-step explanation:
Use Pythagorean theorem.
Assign the largest value to 'c'
If a² + b² = c² , then it is right triangle
If a² + b² > c², then it si acute triangle
If a² + b² < c² , then it is obtuse triangle
Here, a = 9 , b = 12 & c = 18
a² + b² = 9² + 12²
= 81 + 144
a² + b² = 225
c² = 18²
c² = 324
a² + b² < c²
225 < 324
So, 9, 12 , 18 form a obtuse triangle