2)
2(x + 2) + 3x + 1
2x + 4 = 3x + 1
4 - 1 = 3x - 2x
x = 3
4)
25(2a + 3b) = 44(5a - b)
50a + 75b = 220a - 44b
75b + 44b = 220a - 50a
119b = 170a
a/b = 119/170
6)
21/(2x + 1)
x = 0 => 21/(0 + 1) = 21 /1 = 21
x = 1 => 21/(2 + 1) = 21/3 = 7
x = 2 => 21/5 ∉ N
x = 3 => 21/7 = 3
x = 4 => 21/9 ∉ N
x = 5 => 21/11 ∉ N
x = 6 => 21/13 ∉ N
x = 7 => 21/15 ∉ N
.......
x = 10 => 21/21 = 1
A = {1, 3, 7, 21}
9)
BOC + COD + AOD = 360 - AOB
(x + 15) + (x + 15) + 3x = 360 - 90
5x + 30 = 270
5x = 270 - 30
5x = 240
x = 48°
COD = 48° + 15° = 63°
AOD = 3×48° = 144°
