1、 [f^(π/4-x)+f^(π/4+x)]/2 =[cos^(π/4-x)+cos^(π/4+x)]/2 =[(cos(π/2-2x)+1)/2+ (cos(π/2+2x)+1)/2]/2 =[sin2x+1+ (-sin2x+1)]/4 =2/4 =1/2 f^(π/4-x)f^(π/4+x) =cos^(π/4-x)cos^(π/4+x) =[(cos(π/2-2x)+1]/2×[(cos(π/2+2x)+1]/2 =(1+sin2x)(1-sin2x)/4 =(1-sin^2x)/4 =cos^2x/4≤1/4
如图所示: