c = a + b
c = (a2 + b2 + 2 a b cosα)1/2
a b = a b cosα
c = |a x b| = a b sinα
a = ax i + ay j
a = (ax2 + ay2)1/2
cx = ax + bx
cy = ay + by
a b = ax bx + ay by
a x b = (ax by - ay bx) k