Answer:
BC = 29.4
Step-by-step explanation:
30² = 20² + BC² -2•20•BC•cos(108)
900 = 400 +BC² -12.4•BC
900 -400 = BC² -12.4•BC
BC² -12.4•BC -500 = 0
Solve the equation using Bhaskara and you'll find two possible values for BC, which are:
BC = -17 and BC = 29.4
A negative number doesn't make sense because it is a side of a triangle, so we choose BC = 29.4
