中四maths

1)
1/a=(b-c/1)d
不明白??

1/a= (b-c)d
1/ad = b- c
c = b- 1/ad =(abd -1)/ad

2)
﹝(1-cos@)(1+cos@)﹞/tan^2@
= (1 - cos^2 @)/tan^2@
= sin^2 @/tan^2@
=sin^2 @/(sin^2 @/ cos^2 @)
=sin^2 @(cos^2 @/ sin^2 @)
=cos^2 @

3)
(x+y)^2-(r-s)^2
=[x+y+(r-s)][x+y-(r-s)]
=(x+y+r-s)(x+y-r+s)

4) cube have 9 planes of refection
5) cube have 13 axis of rotation
6) REGULAR tetrahedron have 6 planes of reflection
7) REGULAR tetrahedron have 7 axis of rotation