What is the missing reason in step 8? pythagorean theorem definition of cosine substitution properties of multiplication8.a2  =  b2  – 2bccos(a) +  c2            8.?

Pythagorean theorem is used only for the right angled triangles.

For ΔABC,

Cosine rule is

Here we have used one of these formulae

Hence definition of cosine or Cosine rule is the right answer

Option 2) is the right answer

you need a common difference between consecutive terms to be constant, like 1,2,3,4 or 2,4,6,8, or 1,3,6,9

b is the answer that makes most sense

step-by-step explanation: