問39
(1)
a / sin A = b / sin B = c / sin C = 2R
c = 2R sin C
= 2R sin ( π - (A+B) )
= 2R sin (A+B)
= 2R (sin A cos B + cos A sin B)
= 2R( ±sin A sqrt{1-sin^{2}B} + sqrt{1-sin^{2}A} sin B)
= 2R ( ±a/2R * sqrt{1-b^[2}/4R^{2}} + sqrt{1 - a^{2}/4R^{2} * b/2R )
= ±a sqrt{4R^{2} - b^{2} }/2R + b sqrt{4R^{2} - a^{2} }/2R
=( b sqrt{4R^{2} - a^{2}} ± a sqrt{4R^{2} - b^{2}} )/2R
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