求解两道高中数学题？谢谢~~

Question

1. cos20*cos40*cos80=
2.已知Sn是等差数列{an}的前n项和，a2：a4=7：6，S7：S3=

Answer 1

1.
cos20cos40cos80
上下同时乘以8sin20,得：
=8sin20cos20cos40cos80/(8sin20)
=4sin40cos40cos80/(8sin20)
=2sin80cos80/(8sin20)
=sin160/(8sin20)
因为sin160=sin(180-160)=sin20,所以：
=sin20/(8sin20)
=1/8

2. S7/S3
=[7(a1+a7)/2]/[3(a1+a3)/2]
=(7/3)*[(a1+a7)/(a1+a3)]
由于: {an}是等差数列
1+7=2*4, 1+3=2*2
则: a1+a7=2a4,a1+a3=2a2
则有: S7/S3
=(7/3)*[(a1+a7)/(a1+a3)]
=(7/3)*[2a4/2a2]
=(7/3)*[a4/a2]
=(7/3)*[6/7]
=2

Answer 2

1=1/8
2 =2