数学！函数！步骤'急

显示全部楼层 · 2012-4-13 10:40:12

cosa=-3／5，a∈（π／2，π），所以sina>0sina=4/5cos(a+π/3)=cosacos(π/3)-sinasin(π/3)=(-3/5)*(1/2)-(4/5)*(√3/2)=-3/10-4√3/10 =-3/10-2√3/5...

千问 · 2012-4-13 10:40:12

由a∈（π／2，π）可知sina=4/5,还有已知的cosa=-3／5，根据公式，cos（a+π/3）=cosacosπ/3 -sinasinπ/3;答案自己求~~...

千问 · 2012-4-13 10:40:12

cosa = -3/5, sina = 4/5cos(a+π/3) = cosa cos(π/3) - sina sin(π/3)= (-3/5) * (1/2) ﹣(4/5) * (√3/2)= ﹣(3+4√3)/10...