Suppose someone tells you that she has a triangle with sides having lengths 2.6, 8.1, and 8.6. is this a right triangle? why or why not? is there an angle in the triangle larger than 90 degree?

First you check for Pythagoras'  Theorem.

Is  8.6² = 8.1²  + 2.6²  ? 
  
8.1²  + 2.6² = 65.61 + 6.76 = 72.37
8.6² =  73.96

8.1²  + 2.6²  ≠ 8.6² 

Since Pythagoras' Theorem is not satisfied it is not a right angled triangle.

The longest side is 8.6,  let's find the angle facing the longest side 8.6.

By cosine formula  CosA =  (b² + c² - a²) / (2bc)

CosA =  (2.6² + 8.1² - 8.6²) / (2*2.6*8.1) = -1.59/42.12 = -0.0377

A = Cos inverse (-0.0377) = 92.1 degrees

So there is  an angle is greater than 90 degrees.