http://good-times.webshots.com/photo/2964690510103642988OYwWQR
explain plx~
Maths - M.C.
2009-04-26 7:59 pm
回答 (1)
2009-04-26 8:13 pm
✔ 最佳答案
cos ( 270 - x) = cos [180 + (90 - x)] = - cos (90 - x) = - sin x.cos ( x - 270) = cos [ - (270 - x)] = cos ( 270 - x) = - sin x.
So the expression becomes:
(cos x - sin x)/[ cos ^ 2 x - (- sin x)^2]
= ( cos x - sin x)/(cos^2 x - sin^2 x)
= ( cos x - sin x)/(cos x - sin x)(cos x + sin x)
= 1/(cos x + sin x).
收錄日期: 2021-04-25 22:38:38
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20090426000051KK00582