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x in (-oo:+oo)
y-4 = (1/2)*(x-15) // - (1/2)*(x-15)
y-((1/2)*(x-15))-4 = 0
(-1/2)*(x-15)+y-4 = 0
y-1/2*(x-15)-4 = 0
y-1/2*(x-15)-4 = 0
y-1/2*x-4+15/2 = 0
y-1/2*x+7/2 = 0
y-1/2*x+7/2 = 0
y-1/2*x+7/2 = 0 // - y+7/2
-1/2*x = -(y+7/2) // : -1/2
x = (-(y+7/2))/(-1/2)
x = 2*(y+7/2)
x = 2*(y+7/2)
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