LAW OF COSINES

a = 4; b = 8; c = 11

c² = a² + b² - 2ab cos(C)

11² = 4² + 8² - 2*4*8 * cos(C)

121 = 16 + 64 - 64 * cos(C)               Sqare values

121 - 16 - 64 = -64 * cos(C)              Subtract a and b from both sides

41 = -64 * cos(C)                             

41 / -64 = cos(C)                               Divide both sides by -2ab

C = cos-1(-0.641)                              Inv cos

Repeat for 1 more angle then use Pythag for the third

Or, for the third, just use the fact that the three angles must sum to 180°

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