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7 years, 8 months ago
7 years, 5 months ago
From cosine formula, we know that
a^2 = b^2 + c^2 - 2cosB
Similarly, we know that
b^2 = a^2 + c^2 - 2cos2B
and
c^2 = b^2 + a^2 - 2cos(180 - 3B)
Solving these 3 equations, we can get the desired equation.
Hope that helps.