The vertices of a triangle ABC are A(0, 0), B (2, -1) and C (9, 0). Find cos B.
Key points to solve the problem:
• Idea of distance formula- Distance between two points P(x1,y1) and Q(x2,y2) is given by- PQ = 
Given,
Coordinates of triangle and we need to find cos B which can be easily found using cosine formula.
See the figure:
From cosine formula in ΔABC , We have:
cos B = 
using distance formula we have:
AB = 
BC = 
And, AC = 
∴ cos B = 
1