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[知識] 問數.(自量)

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cos(x)=12/13 so sin(x)=5/13
cos(x+y)=140/221 so sin(x+y)=171/221   自己用畢氏定理 就計得番.
sin(y)=sin(x+y-x)=sin(x+y)cos(x)-cos(x+y)sin(x)=
(171/221)(12/13)-(140/221)(5/13)=(2052/2873)-(700/2873)=0.470588235=(8/17)
破解完畢

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