keke_1314
幼苗
共回答了15个问题采纳率:93.3% 举报
你写错了,是cos(a+b)=cosacosb-sinasinb.
推导如下:
cos(a-b)
=cos[a+(-b)]
=cosacos(-b)-sinasin(-b)
=cosacosb+sinasinb
sin(a+b)
=cos[π/2-(a+b)]
=cos[(π/2-a)-b]
=cos(π/2-a)cosb+sin(π/2-a)sinb
=sinacosb+cosasinb
sin(a-b)
=sin[a+(-b)]
=sinacos(-b)+cosasin(-b)
=sinacosb-cosasinb
tan(a+b)
=sin(a+b)/cos(a+b)
=(sinacosb+cosasinb)/(cosacosb-sinasinb) 分子分母同除cosacosb
=(tana+tanb)/(1-tanatanb)
tan(a-b)
=sin(a-b)/cos(a-b)
=(sinacosb-cosasinb)/(cosacosb+sinasinb) 分子分母同除cosacosb
=(tana-tanb)/(1+tanatanb)
sin2a=sin(a+a)=ainacosa+cosasina=2sinacosa
cos2a
=cos(a+a)
=cosacosa-sinasina
=(cosa)^2-(sina)^2
=2(cosa)^2-1
=1-2(sina)^2
tan2a=tan(a+a)=(tana+tana)/(1-tanatana)=2tana/[1-(tana)^2]
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1年前
追问
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举报
keke_1314
cosa=cos[2(a/2)]=1-2(sina/2)^2、则sina/2=+-√[(1-cosa)/2] cosa=[2(a/2)]=2(cosa)^2-1、则cosa/2=+-√[1+cosa)/2] tana/2=(sina/2)/(cosa/2)=+-√[(1-cosa)/(1+cosa)] (1-cosa)/(1+cosa)=(tana/2)^2,解得:cosa=[1-(tana/2)^2]/[1+(tana/2)^2] sina=cosatana=[1-(tana/2)^2]/[1+(tana/2)^2]*(2tana/2)/[1-(tana/2)^2]=2(tana/2)/[1+(tana/2)^2]