281990177
幼苗
共回答了9个问题采纳率:100% 举报
tan(9π/4+a)*tan(3π/4+a)
=tan(2π+π/4+a)*tan(π-π/4+a)
= tan(a+π/4)*tan(a-π/4)
=(1+tana)/(1-tana)*(1-tana)/(1+tana)
=1
sina=sin(a+30-30)=√3/2*sin(a+30)-1/2cos(a+30)
60<a<90
90<a+30<120
cos(a+30)=-5/13
√3/2*sin(a+30)-1/2cos(a+30)
=√3/2*12/13+1/2*5/13
=(12√3+5)/26
所以sina=(12√3+5)/26
1年前
追问
10
举报
281990177
由tan(π/4-a)=1/3得 tana=1/2 {2cosa(sina-cosa)}/1+tana =[2cos^2a(1/2-1)/(1+1/2) =-2/3cos^2a tana=1/2构成一直角三角形得 cos^2a=4/5 所以-2/3cos^2a=-8/15