Given the right triangle abc, with c the right angle and legs ac and bc measuring 6 and 8, respectively, what is cos b?

Given that the formula is Cos θ = adj/hyp. We cannot apply the formula until we know what is the length of the hypotenuse. 

To find hypotenuse, we will use the Pythagoras theorem:

a² + b² = c²

Substitute a = 6 and b = 8, we will be able to find c.

6² + 8² = c²
c² = 36 + 64
c² = 100
c = √100
c = 10

Now that we know that the hypotenuse is 10 units, we can find cos B:

Cos B = adj/hyp
Cos B = 8/10
Cos B = 4/5


Cos B = 4/5

A quadrilateral has four sides meaning it will also have four angles that all add up to 360 degrees. Since you already know three of the angles (60, 45 and 100) you just need to add them up and subtract the answer from 360.

60+45+100=205
360-205=155

So the fourth angle is 155 degrees