大师作文网设a为锐角,若cos(s 六分之派)=五分之四
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已知sinA=四分之三,cos（A+B）=负三分之二,A,B为锐角,那么：0再问：大神！为什么sinb=sin（a+b-a）=sin（a+b）cosa-cos（a+b）sina？（不好意思，我数学底子
∵a为锐角,∴π/4
因为cos(α+π/4)=3/5,所以sin(α+π/4)=4/5.所以sinα=sin[(α+π/4)-π/4]=sin(α+π/4)cos(π/4)-cos(α+π/4)sin(π/4)=√2/2
用二倍角公式再问：答案。我知道公式不会写再答：我拍给你再答：再答：希望采纳
a为锐角,cos(a+π/6)=4/5所以,sin(a+π/6)=3/5sin(2a+π/3)=2sin(a+π/6)cos(a+π/6)=2×(3/5)×(4/5)=24/25cos(2a+π/3)
设b=a+π/6,sinb=3/5,sin2b=2sinbcosb=24/25,cos2b=7/25sin（2a+π/12）=sin（2a+π/3-π/4）=sin（2b-π/4）=sin2bcosπ
cos(2a+π/3)=2cos²(a+π/6)-1=7/25a为锐角,则：2a+π/3∈(π/3,π/2)∴sin(2a+π/3)=24/25sin(2a+π/12)=sin[(2a+π/
50分之17倍根号2
a为锐角,cos(a+π/6)=3/5则：sin(a+π/6)=4/5sin(a-π/12)=sin[(a+π/6)-π/4]=sin(a+π/6)cos(π/4)-cos(a+π/6)sin(π/4
sin（2a+π/12）=sin【2(a+π/6)—π/4】=sin2(a+π/6)cos(π/4）—cos2(a+π/6)sin(π/4）因为cos(a+π/6)=4/5,a为锐角,所以0
设A+π/6=α,则2A+π/12=2α-π/4cosα=4/5,α为锐角,sinα=3/5,所以sin2α=2sinαcosα=24/25,cos2α=2(cosα)^2-1=7/25sin(2A+
a为锐角,cos(a+π/6)=3/5则：sin(a+π/6)=4/5sin(a-π/12)=sin[(a+π/6)-π/4]=sin(a+π/6)cos(π/4)-cos(a+π/6)sin(π/4
a//b,可得：a=tb,所以有：3=√3t解得：t=√3sina=cosat所以：sina/cosa=t即：tana=√3 得：a=60°
1、tana=1/2=sina/cosa2sina=cosa4sin^2a=cos^2a4(1-cos^2a)=cos^2acosa=2倍根5/5（因为a为锐角）2、Sin2aCosa-Sina/Si
设sina-cosa=t则2sinacosa=1-t^21-t^2+(1/3)t=13t^2-t=0t=0或t=1/3sinacosa=1/2或4/9tana+cota=1/sinacosa=9/4t
sinα+cosα=4/3两边平方(sinα+cosα)=16/9sin²α+cos²α+2sinαcosα=16/91+2sinαcosα=16/92sinαcosα=7/9si
a∈(0,90)a+30∈(30,120)sin(a+30)>0sin(a+30)=3/5sin(2a+60)=2sin(a+30)cos(a+30)=24/25又cos(a+30)>cos45,a+