作业帮cos2a/sin(a-π/4) =-根号（2）/2,则cosa+sina的值为?
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cos2a/sin(a-π/4) =-根号（2）/2,则cosa+sina的值为?
cos2a/sin(a-π/4) =-根号（2）/2,则cosa+sina的值为?
cos2a/sin(a-π/4) =-根号（2）/2,则cosa+sina的值为?
cos2a/sin(a-π/4)=(2cos2a*cos(a-π/4)) /(2sin(a-π/4) cos(a-π/4))
cos2a/sin(a-π/4)=(2cos2a*cos(a-π/4)) /sin(2a-π/2)
cos2a/sin(a-π/4)=-(2cos2a*cos(a-π/4)) /cos2a
cos2a/sin(a-π/4)=-2*cos(a-π/4)
-2*cos(a-π/4) =-√2/2
cos(a-π/4) =√2/4
cosa*cos(π/4)+sina*sin(π/4)=√2/4
cosa+cosb=1/2
sin[a-(pi/4)]=(sina-cosa)/(根号2)
所以cos2a/sin[a-(pi/4)]=(根号2)cos2a/(sina-cosa)
=(根号2)[(cosa)^2-(sina)^2]/(sina-cosa)
=-(根号2)(cosa+sina)=-(根号2)/2
cosa+sina=1/2
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cos2a/sin(a-π/4) =-根号（2）/2,则cosa+sina的值为?