✔ 最佳答案
1.呢題我用左兩邊一齊計既方法計因為呢條數originally from sin^2+cos^2=1
sin^2 x+cos^2 x= 1 呢度左右都要除以cosxsinx
sinx/cosx + cosx/sinx =1/cosxsinx
tanx + cotx =1/(1/2)sin2x
=2/(sin2x)
tanx + cotx =2cosec2x
2. 呢條我只係用左邊計
cos3x = cos(2x+x) = cos2x cosx - sin2x sinx
= (cos^2 x- sin^2 x)cosx - 2sin^2 xcosx
= cos^3 x- sin^2 xcosx - 2sin^2 xcosx
= cos^3 x- 3sin^2 xcosx 呢度用sin^2 x =1-cos^2 x
= cos^3 x- (3cosx - 3cos^3 x)
= 4cos^3 x - 3cosx
3.呢條都係用左邊計
cosec2x - cot2x
= (1/sin2x) - (cos2x/sin2x)
= (1-cos2x)/sin2x 呢度再用cos2x = cos^2 x - sin^2 x
= (1-[cos^2 x - sin^2 x]) / sin2x 再用1= cos^2 x + sin^2 x
= ([sin^2 x + cos^2 x] - [cos^2 x - sin^2 x] ) / sin2x
= 2sin^2 x / sin2x
= 2sin^2 x / 2sinxcosx
= sinx/cosx
=tanx
以上幾個prove因為我怕解得唔清晰so每一step都寫左出黎所以有d長=.=
答問題我估4個step倒就夠...
2007-09-09 07:05:22 補充:
IVANCYP 第一題答案你打少左野= ( sin^X + cos^X ) / (sin X cos X) <-----(通份母)呢度你唔記得打sin^2 同cos^2