Every point in the unit circle is identified either by its coordinates 
or by the angle it forms with the x-axis, 
.
The trigonometric functions associate with every angle and the correspondant coordinates the two values
This procedure can be done for every angle , so you don't have to work with acute angles only.
Answer:
Let x = the charge in 1st city before taxes
Let y = the charge in 2nd city before taxes
Set up equation before taxes.
y = x - 1500 eq1
Set up equation for total tax paid.
0.065x + 0.06y = 378.75 eq2
Substitute eq1 into eq2.
0.065x + 0.06(x - 1500) = 378.75
0.065x + 0.06x - 90 = 378.75
0.125x - 90 = 378.75
0.125x = 468.75
x = 3750
Substitute this value of x into eq1.
y = 3750 - 1500
y = 2250
The hotel charge in city one is $3750 and the hotel charge in city two is $2250
The equation of the line that passes through the points
(1,5) and (5,13) is y=2x+3
