algebra
posted by Anonymous boy on .
in triangle ABC, the size of angle B is 5 times the size of angle A and the size of angle C is 9 degrees less than 4 times the size of angle A
Find the size of angle A

You can set up the equations as follows:
B=5A
C=4A9
A+B+C=180 (sum of angles of a triangle).
and solve for A, B and C.
However, you can easily solve it by substitution into the third equation:
A+B+C=180
A + 5A + 4A9 = 180
10A=189
A=18.9
With A known, you can then find the remaining angles by substitution into the first two equation. 
The sum of the angles of a triangle are 180 degrees.
Therefore:
Let x = angle A
5x = angle B
4x  9 = angle C
Equation:
x + 5x + 4x  9 = 180
Solve for x.