The term that best describes a proof in which you assume the opposite of what you want to prove is proof of contradiction. Contradiction means opposite.
Your answer is: C) Proof of Contradiction
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Answer:
The third side is around 58.043
Step-by-step explanation:
Use the law of cosines: 
Plug in the two sides we know (into a and b) and the angle we know (into angle C).
Thus:
Use a calculator:


(Note: Make sure you're in Degrees mode.)
Step-by-step explanation:
a. x = 30° (180-70-80)
b. <FGA = 80°
c. vertical angles
Answer:
I think the answer is 42.