小pig
幼苗
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∵ (cosB)^2+(sinB)^2=1,(cosC)^2+(sinC)^2=1
∴ (sinB)^2=1-(cosB)^2=1-(-5/13)^2=1-25/169=144/169
(sinC)^2=1-(cosC)^2=1-(4/5)^2=1-16/25=9/25
从而 sinB=12/13,sinC=3/5
在三角形ABC中,A+B+C=180°
∴ A=180°-B-C
从而 sinA=sin(180°-B-C) [三角函数公式:sin(π-α)= sinα }
=sin(B+C)
=sinBcosC+cosBsinC
=12/13*4/5+(-5/13*3/5)
=48/65-15/65
=33/65
1年前
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