Pythagorean theorum equation is
That basically means that in a right triangle(has a 90 degree angle), you add up the sides squared, that will give you the diagonal number squared. To find the hypotenuse(diagonal), just plug that number in a square root.
You can use this equation to find sides too, by doing the same process, only subtracting the side you have by the hypotenuse:
right angle = has a 90 degree angle
Obtuse angle= has an angle larger than 90 degrees
Acute angle= all angles are smaller than 90 degrees
In section 7.3, in the diagram you can assume that angle bdc is 90 degrees because a straight line is 180, and it is divided with 90 degrees and will need another 90 degrees to complete a line. That makes that triangle a right angle, and you can use the pythagorean theorum to find the side lengths.
Hope this helps!
