Solution: The missing reason in Step 8 is substitution of .
Explanation:
The given steps are used to prove the formula for law of cosines.
From step 5 it is noticed that our equation is
..... (1)
From step 7 it is noticed that the value of is
.
So by substituting for
in equation (1) we get the equation of step 8, i.e.,
Hence, the missing reason in Step 8 is substitution of .