三角比 F.4

(1 - tan^2 x) / (1 + tan^2 x) + sin^2 x

= [1 - (sin^2 x)/cos^2 x)] / [1 + (sin^2 x)/(cos^2 x)] + sin^2 x

= [(cos^2 x - sin^2 x)/cos^2 x] / [(sin^2 x + cos^2 x) / cos^2 x] + sin^2 x

= (cos^2 x - sin^2 x) / (sin^2 x + cos^2 x) + sin^2 x

= (cos^2 x - sin^2 x) / 1 + sin^2 x

= cos^2 x

2009-10-18 18:31:08 補充：
= (cos^2 x - sin^2 x) / (sin^2 x + cos^2 x) + sin^2 x 點變

= (cos^2 x - sin^2 x) / 1 + sin^2 x

因為 sin^2 x + cos^2 x = 1

注意 (cos^2 x - sin^2 x) / 1 + sin^2 x 不要理解成

(cos^2 x - sin^2 x) / (1 + sin^2 x )

2009-10-18 18:32:13 補充：
= (cos^2 x - sin^2 x) / 1 + sin^2 x

= cos^2 x - sin^2 x + sin^2 x

= cos^2 x

這裏可以幫到你
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