A triangle is 180°, so the sum of the three angles should be 180°
To find "x", you can do this:
#1
39° + 31° + (11x + 11)° = 180°
39 + 31 + 11x + 11 = 180 Combine like terms
81 + 11x = 180 Subtract 81 on both sides
11x = 99 Divide 11 on both sides
x = 9
#2
45° + 45° + (4x + 18)° = 180°
45 + 45 + 4x + 18 = 180 Combine like terms
108 + 4x = 180 Subtract 108 on both sides
4x = 72 Divide 4 on both sides
x = 18
You could also do this:
This is an isosceles triangle (a triangle with 2 congruent/equal sides and 2 congruent angles). Specifically, it is a isosceles right triangle with angles 45, 45, and 90. Since you know the angle is 90°, you can do this:
(4x + 18)° = 90°
4x + 18 = 90 Subtract 18 on both sides
4x = 72 Divide 4 on both sides
x = 18
