-12n+12m that is the answer
By definition, skew lines are a pair of lines that are not parallel but do not intersect each other either. Hence, the conclusion would be, the lines are non-coplanar. An example would two random lines drawn in the x and y axes in a cartesian plane, respectively.
Answer:

Step-by-step explanation:
we know that

Remember the identity

step 1
Find the value of 
we have that
The angle alpha lie on the III Quadrant
so
The values of sine and cosine are negative

Find the value of sine

substitute




step 2
Find the value of 
we have that
The angle beta lie on the IV Quadrant
so
The value of the cosine is positive and the value of the sine is negative

Find the value of cosine

substitute




step 3
Find cos (α + β)

we have




substitute



The graph will be as shown in attached figure