cos^2a-cos2acos4a=sin^2 3a

cos^2a-cos2acos4a=sin^2 3a 求证：cos^2a-cos2acos4a=sin^2 3a 求证:cos^2a-cos2acos4a=sin^2 3a 化简：[cos(a+15度)]^2+[sin(a-15度)]^2+[sin(a+180度)]*[cos(a-180度)].请大家解释一下.[cos(a+15度)]^2+[sin(a-15度)]^2+[sin(a+180度)]*[cos(a-180度)]=[cos(2a+30)+1]/2+[1-cos（2a-30)]/2+1/2sin2a=1+[cos(2a+30)-cos(2a-30)]/2+1/2sin2a=1-2si 5cos^2a+4cos^2β=4cosα则cos^2a+cos^2β 化简(1-sin^6 a-cos^6 a)/(cos^2 a-cos^4 a)== sin(α+π/3)+sinα=负5分之4根号3 α∈（-π/2,0）求cosα怎样从9/4*sin²α=9/4(1-cos²α)=3/4*cos²a+12/5*cosx+48/25化为cos²α+4/5*cosα-11/100=0呀sin(α+π/3)+sinα=-4√3/5sinαcosπ/3+cosαsinπ/3+sinα=-4√3/53/2*si 2cos^2(a)-cos(a)-1=0 cos^2(a+b)=? 已知(1+sina)/cosa=-1/2,则cosa/(sina-1)的值是?已知(1＋sina)／cosa＝－1／2而sin²a+cos²=1,1-sin²a=cos²,(1-sina)(1+sina)=cos²a所以cosa／(sina－1)＝ -(1＋sina)／cosa=1/2为什么cosa可以直接换成 -(1＋si 求证:a^2(cos^2b-cos^2c)+b^2(cos^c-cos^2a)+c^2(cos^2a-cos^2b)=0 求证cos(a+b)cos(a－b)=cos^2b－sin^2a 证明cos(a+β)cos(a-β)=cos^2a-sin^2β 求证cos(a+b)cos(a-b)=cos^2a-sin^2b Sina+sin^2a=1,cos^2a+cos^s4a+cos^6a 化简:cos^2a-cos(a+60)cos(a-60)= 证明：cos平方A+cos平方(60-A)+cos平方(60+A)=3/2 cos (2 a/7）+cos(4 a/7)+cos(6 a/7)=?