# student2280

## 解答 0

• eric
2019-05-22 20:54:49
21. Proof is given in https://zhidao.baidu.com/question/53815592.html
• eric
2019-05-22 20:59:06
25a. Use sin(90°-θ)=cosθ
• student2280
2019-05-22 21:34:03
Thanks
• tayh
2019-05-23 00:46:10
25(b) y= sin20sin40sin80*(sin60) =-1/2(cos60-cos20)sin80*(根号3/2) =-1/2(1/2-cos20)sin80*(根号3/2) =-1/4*sin80+1/2*[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*sin80+1/4*sin60-1/4*sin80)*根号3/2 =1/4*根号3/2*根号3/2 =3/16 so, y sin 10 = 3/16 sin 10 = 3/16 sin 80

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• tayh
2019-05-23 00:49:47
更正 25(b) y= sin20sin40sin80*(sin60) =-1/2(cos60-cos20)sin80*(根号3/2) =-1/2(1/2-cos20)sin80*(根号3/2) =-1/4*sin80+1/2*[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*sin80+1/4*sin60+1/4*sin80)*根号3/2 =1/4*根号3/2*根号3/2 =3/16 so, y sin 10 = 3/16 sin 10 = 3/16 sin 80

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