【Genius is one percent inspiration and ninety-nine percent perspiration】--- 爱迪生
【Without the one percent of inspiration, all the perspiration in the world is only a bucket of sweat】 ---美国作家Cindi Myers
已知$cosxcos2xcos3x+sinxsin2xsin3x=1$，求$x$=———
解答：记$max{|cos2x|,|sin2x|}=tle1$
$1=cosxcos2xcos3x+sinxsin2xsin3x$
$le (|cosxcos3x|+|sinxsin3x|)tle max{|cos2x|,|cos4x|}le |cos2x|le1$
易知等号要成立:$x=kpi,kin Z$
注:$|cos4x|le |cos2x|$