Well, first I need to explain A for you to understand B.
So, an isosceles triangle has two equal angles. Assuming that the third angle is obtuse, it has to be greater than 90 degrees.
Now, do 180-91. that equals 89. 89 divided by two equals 44.5. This is an acute angle, and 44.5+44.5+89=180. 180 is the sum of the angles of a triangle
For B, it's honestly just a description of the way you did it. You can use what I did or if you did a different method.
Sorry it's so late! I hope it's on time!
