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1.s=c^2-(a-b)^2=c^2-(a^2+b^2)+2ab (所有的项拆开) (余弦定理是突破点) -s=a^2+b^2-c^2-2ab ,s=1/2absinC (1) cosC=a^2+b^2-c^2/2ab (2) (1)代入(2),ab约去, 4cosC=sinc-4, (3) sinC^2+cosC^2=1 (4) (3)(4)联立,求出cosC=-1(舍),或-15/17 所以,sinC=8/17tanC=(8/17)/(-15/17)=-8/15(2)S=1/2absinC32/17=1/2ab*8/17ab=8 |
