The lengths of the sides of a triangle are 6, 8, 10. can the triangle be a right triangle? yes or no?

  see the attachment

Step-by-step explanation:

It helps to follow along the proof and consider how you get from here to there.

In the 5th line, we have a sum in symbols.

In the 7th line, we have an expression with a number on the right side, and a statement that says we got there by subtraction.

Then we expect line 6 to have a sum with numbers whose difference is 40°. (That eliminates the 3rd and 4th choices.)

The way you get from an expression with symbols to an expression with numbers is to make a substitution of numbers for symbols. Hence the appropriate choice is the one shown below.