a/sinA=b.sinB=c/sinC=2R∴sinA=a/2R,sinB=b/2R,sinC=c/2R∴2R(a²/4R-c²/4R²）=(√2a-b)b/2R∴a²-c²=(√2a-b)b=√2ab-b²∴a²+b²-c²=√2ab余弦定理 cosC=(a²+b
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