Midpoint = ((x₁+x₂)/2, (y₁+y₂)/2)
d = √((x₂-x₁)²+(y₂-y₁)²)
m = (y₂-y₁)/(x₂-x₁)
ax+by+c=0 → m = -a/b
Parallel: equal · Perp: m₁m₂=-1
y - y₁ = m(x - x₁)
Two points? Find the slope first
x=0 → y-intercept
y=0 → x-intercept · join them
Area = ½|x₁(y₂-y₃)+x₂(y₃-y₁)+x₃(y₁-y₂)|
Circle: x²+y²=r²