Answer: 
<u>Step-by-step explanation:</u>

False- the included angle must be a right angle for cos C to be 0 & two other points are normally used
Let us find the slopes of the two lines
ax+by=c, slope m1 = -a/b
bx-ay=c, slope m2 = b/a
m1*m2 = (-a/b)(b/a) = -1
Therefore the two lines are perpendicular so the answer is D