sin20sin40sin80求值...

sin20sin40sin80*(sin60)=-1/2(cos60-cos20)sin80*(根号3/2)=-1/4*sin80+1/2*[1/2*(sin60+sin100)]*根号3/2 =(-1/4*sin80+1/4*sin60+1/4*sin100)*根号3/2 =1/4*根号3/2*根号3/2 =3/16

积化和差2sinasinb=cos(a-b)-cos(a+b),2sinacosb=sin(a+b)+sin(a-b).sin20sin40sin60sin80 =[(cos20-cos60)sin80]sin60/2 =[(sin100+sin60)/2-sin80/2]sin60/2 =(sin60/2)^2 =3/16.

=sin40°cos40°cos80°/(2sin20°)=sin80°cos80°/(4sin20°)=sin160°/(8sin20°)=sin20°/(8sin20°)=1/8 诱导公式口诀“奇变偶不变，符号看象限”意义：k×π/2±a(k∈z)的三角函数值。(1)当k为偶数时，等于α的同名三角函数值，前面加上一个把α看作锐角时原三角函数值的...

sin20sin40sin80*(sin60)=-1/2(cos60-cos20)sin80*(根号3/2)=-1/4*sin80+1/2*[1/2*(sin60+sin100)]*根号3/2 =(-1/4*sin80+1/4*sin60+1/4*sin100)*根号3/2 =1/4*根号3/2*根号3/2 =3/16

因为sin20°sin40°sin80° =(cos10°cos70°)cos50° =(1/2)(cos80°+cos60°)cos50° =(1/2)cos80°cos50°+(1/4)cos50° =(1/4)(cos130°+cos30°)+(1/4)cos50° =(1/4)cos(180°-50°)+(√3/8)+(1/4)cos50° =(√3/8)+(1/4)cos50°-(1/4)cos50° =...

sin28=0.4694715627858908 sin29=0.48480962024633706 sin30=0.49999999999999994 sin31=0.5150380749100542 sin32=0.5299192642332049 sin33=0.544639035015027 sin34=0.5591929034707468 sin35=0.573576436351046 sin36=0.5877852522924731 sin37=0.6018150231520483 sin38=0.6156614753256583 sin39=0.6293203910498375 sin40=0.6427876096865392...

有一个公式 sinθsin(60-θ)sin(60+θ)=1/4sin3θ 所以=sin60(sin20sin(60-20)sin(60+20))=sin60^2/4 =3/16 不然就要先积化合差，再和差化积 sin20sin40sin60sin80 =sin60[0.5sin20(cos40-cos120)]=sin60(0.5sin20cos40+0.25sin20)=sin60(0.25sin60-0.25sin20+0....

sinasin(60-a)sin(60+a)=1/4sin3a

原式=2sin20cos20cos40cos80/2sin20 =sin40cos40cos80/2sin20 =2sin40cos40cos80/4sin20 =sin80cos80/4sin20 =2sin80cos80/8sin20 =sin160/8sin20 =sin(180-160)/8sin20 =sin20/8sin20 =1/8

悬赏200分又不想确认满意答案，投机的好方法！不过本人不在意悬赏分。下面三种解题方法，将字母换过即可。其中第三种解法简单明了。解法1 解法2 解法3