若sin(45-x)= -12/13,且135
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若sin(45-x)= -12/13,且135
cos(45+x)
=sin[90-(45+x)]
=sin(45-x)
而
cos2x=sin(90-2x)=2sin(45-x)*cos(45-x)
所以
cos2x/cos(45+x)
=2sin(45-x)*cos(45-x) / sin(45-x)
=2cos(45-x)
=2cos(x-45)
而sin(45-x)= -12/13,且135
=sin[90-(45+x)]
=sin(45-x)
而
cos2x=sin(90-2x)=2sin(45-x)*cos(45-x)
所以
cos2x/cos(45+x)
=2sin(45-x)*cos(45-x) / sin(45-x)
=2cos(45-x)
=2cos(x-45)
而sin(45-x)= -12/13,且135
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