一條中4數唔識計-'

sinX-cosX=1/4
(sin X - cos Y)^2 = (1/4)^2
(sin X)^2 - 2sinX ．cosX + (cos Y)^2 = 1/16
(sin X)^2 + (cos Y)^2 - 2sinX ．cosX = 1/16
1 - 2sinX cosX = 1/16
1 - 1/16 = 2sinX cosX
15/16 = 2sinX cosX
sinX cosX = 15/32


2010-03-31 23:22:35 補充：
Typing error :

Y should be X

sinX - cosX = 1/4

(sinX - cosX)^2 = 1/16

(sinX)^2 - 2sinXcosX + (cosX)^2 = 1/16

-2sinXcosX = 1/16 - 1

sinXcosX = 15/32

Note : cosX ^2 + sinX ^2 =1

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