【正文】
13. 證明 ∵ )s inac os (a2)a2s in( ?? ??? = )s in ac os (a2a])s in[ (a ?? ???? = ) s in ac os ( a2) s in ac os ( a) c o s as in( a ??? ????? = )s in ac os (a)c os as in( a ?? ??? = a])sin[(a ?? = .sin? 兩邊同除以 【舉一反三、能力拓展】 : ???? c o sc o s)c o s (2c o s]1)(c o s2[21)( 22 xxxxf ?????? = ???? 22 c osc osc os)c os (221)(c os ????? xxx = 21c os]c osc os2)) [ c o s (c os ( 2 ????? ???? xxx = 21c o s]c o sc o ss in) [ s inc o s ( 2 ???? ???? xxx = ??? 2c os21)]c os ()[c os ( ???? xx x2cos21?? ∴ )(xf 的值域為 ]21,21[? ,周期為π,是偶函數(shù), 當 )](2,[ Zkkkx ??? ??? 時 )(xf 是增函數(shù),當 )](,2[ Zkkkx ??? ??