f.3 math (10)~~~~~~~

cos 27*tan27-cos 63

= cos27*(sin27/cos27)-cos63 (tan x=sin x/cos x)
= sin27-cos63
= sin27-sin(90-63) (cos x= sin(90-x))
= sin27-sin27
= 0

2009-04-23 23:14:51 補充：
sin^2 42 + cos^2 48

= sin^2 42+ cos^2 42 (sin^2 x=cos^2(90-x))
=1 (sin^2 x+ cos^2 x=1)

1) (cos 27)(tan27) - (cos 63)
= (cos 27)(sin27/cos27) - (cos 63)
= sin27 - cos27

2) (sin42)^2 + (sin48)
= (sin42)^2 + [sin(90-42)]^2
= (sin42)^2 + (cos42)^2
= 1

3) 4[cos(90-x)][tan(90-X)]
= 4[cos(90-x)][sin(90-X)/cos(90-x)]
= 4sin(90-x)
= 4cosx