求sin(a 75°) cos(a 15°

tana=sina／cosa=2/3.另sina²＋cosa²=1两个方程两个未知数,可解.

答案为:-根号2-m^2方法:sina+cosa=m先平方,得:2sinacosa=m^2-1则:(sina-cosa)^2=1-[m^2-1]=2-m^2又0

[cos(a)-sin(a)]的平方=1-2cos(a)*sin(a)=3/4,cos(a)-sin(a)=根号3/2

sina-4cosa/5sina+2cosa=2cosa-4cosa/10cosa+2cosa=-1/6

sinα-cosα=a等式两边平方可得：sin²α+cos²α-2sinαcosα=a²（其中sin²α+cos²α=1）那么,2sinαcosα=1

-(M-1)=-M+1

cos(a)/u+sin(a)=P[cos(a)/Pu+sin(a)/P](P为常数)令(1/Pu)^2+(1/P)^2=11/P^2u^2+u^2/p^2u^2=1P^2u^2=(1+u^2)P=√

sina=-2cosasin^2a+cos^2a=1(1)sina=2√5/5cosa=-√5/5原式=sina-5/3sina/cosa+sina=4√5/5+10/3(2)sina=-2√5/5c

sin^2(a)+2sin(a)cos(a)+3cos^2(a)/sin(a)cos(a)+cos^2(a)分子分母同除以cos²a,得原式=(tan²a+2tana+3)/(ta

2cos²a+3cosasina-3sin²a=1=sin²a+cos²acos²a+3cosasina-4sin²a=0(cosa+4si

先关注目标...显然..用平方差公式得：cos＾4a－sin＾4a=2cos＾2a-1关键求cos^2a就好...由条件交叉相乘.整理得：sina=2cosa两边平方,联系sin^2a+cos^2a=

原式=（sina/cosa＋1）/（1-sina/cosa）=（tana＋1）/（1-tana）=-3

sina-2cosa=0,则tana=2,则sina=2/根号5,-2/根号5cosa=1/根号5,-1/根号5则cos^2=1/54sin^2a-3sinaxcosa-5cos^2a=1/cos^2

原题是这样子吧：cos(a+b)cos(a-b)=1/5,则(cosa)^2-(sinb)^2=?cos(a+b)cos(a-b)=(cosacosb-sinasinb)(cosacosb+sinas

sin^4(a)+cos^4(a)+sin(360°-a)cos(360°-a)=sin^4(a)+cos^4(a)+sin(-a)cos(-a)=sin^4(a)+cos^4(a)+sin(a)co

cos∧2（-a）-[tan（360°+a）]/sin(-a)=cos²a+tana/sina=cos²a-1/cosa

sina+cosa=m(sina+cosa)^2=m^2(sina)^2+(cosa)^2+2sinacosa=m^21+2sinacosa=m^22sinacosa=m^2-1(cosa-sina)

(1)(sina-3cosa)/(sina+cosa)=(tana-3)/(tana+1)=-2/3(2)(2sin^2a-3cos^2a)/(4sin^2a-9cos^2a)=[2(tana)^2-

sin(α+β)*cos(α-β)=(sinαcosβ-cosαsinβ)(sinαcosβ+cosαsinβ)=(sinα)^2*(cosβ)^2-(cosα)^2*(sinβ)^2=(cosβ)^

依题有2sin2x=sinθ+cosθsinx的平方=sinθ*cosθ又2sin2x=4sinx*cosxsinθ*cosθ=[(sinθ+cosθ)的平方-1]/2所以有sinx的平方=[(4si