Question

What is the missing reason in Step 8? Pythagorean theorem definition of cosine substitution properties of multiplication

Answer 1

Pythagorean theorem is used only for the right angled triangles.

For ΔABC,

Cosine rule is

$a^2=b^2 + c^2-2bc cosA$

$b^2=a^2 + c^2-2ac cosB$

$c^2=a^2 + b^2-2ab cosC$

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