9514 1404 393
Explanation:
The format being used below is ...
(number) (statement) . . . . (reason)
__
1. m∠X = 4a+2; m∠Y = 21a +3 . . . . given
2. ∠X and ∠Y are a linear pair . . . . given
3. m∠X + m∠Y = 180 . . . . definition of linear pair
4. (4a +2) +(21a +3) = 180 . . . . substitution property of equality
5. 25a +5 = 180 . . . . like terms can be combined
6. 25a = 175 . . . . subtraction property of equality (subtract 5)
7. a = 7 . . . . division property of equality (divide by 25)
8 . m∠Y = 21·7 +3 = 150 . . . . substitution property of equality
9. ∠Y is obtuse . . . . definition of obtuse angle
