两个正弦 、余弦定理的题目1.cos a =1/17,cos (a+b)= -47/51 ,a、b属于(0,π/2),求cos b.2.cos (a+b)=4/5,cos (a-b)= -4/5,a+b属于(7π/4,2π),a-b属于(3π/4,π),求cos 2a.谢八辈祖宗!
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![两个正弦 、余弦定理的题目1.cos a =1/17,cos (a+b)= -47/51 ,a、b属于(0,π/2),求cos b.2.cos (a+b)=4/5,cos (a-b)= -4/5,a+b属于(7π/4,2π),a-b属于(3π/4,π),求cos 2a.谢八辈祖宗!](/uploads/image/z/5276411-35-1.jpg?t=%E4%B8%A4%E4%B8%AA%E6%AD%A3%E5%BC%A6+%E3%80%81%E4%BD%99%E5%BC%A6%E5%AE%9A%E7%90%86%E7%9A%84%E9%A2%98%E7%9B%AE1.cos+a+%3D1%2F17%2Ccos+%28a%2Bb%29%3D+-47%2F51+%2Ca%E3%80%81b%E5%B1%9E%E4%BA%8E%EF%BC%880%2C%CF%80%2F2%EF%BC%89%2C%E6%B1%82cos+b.2.cos+%28a%2Bb%29%3D4%2F5%2Ccos+%28a-b%29%3D+-4%2F5%2Ca%2Bb%E5%B1%9E%E4%BA%8E%287%CF%80%2F4%2C2%CF%80%29%2Ca-b%E5%B1%9E%E4%BA%8E%EF%BC%883%CF%80%2F4%2C%CF%80%29%2C%E6%B1%82cos+2a.%E8%B0%A2%E5%85%AB%E8%BE%88%E7%A5%96%E5%AE%97%21)
两个正弦 、余弦定理的题目1.cos a =1/17,cos (a+b)= -47/51 ,a、b属于(0,π/2),求cos b.2.cos (a+b)=4/5,cos (a-b)= -4/5,a+b属于(7π/4,2π),a-b属于(3π/4,π),求cos 2a.谢八辈祖宗!
两个正弦 、余弦定理的题目
1.cos a =1/17,cos (a+b)= -47/51 ,a、b属于(0,π/2),求cos b.
2.cos (a+b)=4/5,cos (a-b)= -4/5,a+b属于(7π/4,2π),a-b属于(3π/4,π),求cos 2a.
谢八辈祖宗!
两个正弦 、余弦定理的题目1.cos a =1/17,cos (a+b)= -47/51 ,a、b属于(0,π/2),求cos b.2.cos (a+b)=4/5,cos (a-b)= -4/5,a+b属于(7π/4,2π),a-b属于(3π/4,π),求cos 2a.谢八辈祖宗!
1.
cosA=1/17,sinA=12根号2/17
cos(A+B)=cosAcosB-sinAsinB
=1/17cosB-12根号2/17sinB=-47/51
根据 cos平方B+sin平方B=1
解方程得 cosB=2根号2/3
2.
sin(α-β)=3/5,sin(α+β)=-3/5
cos2α=cos(α-β+α+β)
=cos(α-β)cos(α+β)-sin(α-β)sin(α+β)
=(-4/5)(4/5)-(3/5)(-3/5)
=-7/25
第二个:先求出sin(a+b)=-3/5 sin(a-b)=3/5 再求cos[(a+b)+(a-b)]就可以了!第一问数字太大,计算的你按计算机吧,我告诉你方法…先求出sina 再求出sin(a+b)再求cos[(a+b)-a]就可以了!
1.cos a =1/17,cos (a+b)= -47/51 , a、b属于(0,π/2),求cos b.
a=arccos(1/7),
a+b=arccos(-47/51),
b=arccos(-47/51)-arccos(1/7),
2.cos (a+b)=4/5, cos (a-b)= -4/5,a+b属于(7π/4,2π),a-b属于(3π/4,π),求...
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1.cos a =1/17,cos (a+b)= -47/51 , a、b属于(0,π/2),求cos b.
a=arccos(1/7),
a+b=arccos(-47/51),
b=arccos(-47/51)-arccos(1/7),
2.cos (a+b)=4/5, cos (a-b)= -4/5,a+b属于(7π/4,2π),a-b属于(3π/4,π),求cos 2a.
sin(a+b)=-3/5,
sin(a-b)=3/5,
cos(a+b)cos(a-b)-sin(a+b)sin(a-b)
=cos(a+b+a-b)
=cos2a
=-16/25-(-9/25)
=-7/25
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