根据cos（2x）=2cos²(x）-1,所以原式=1/2【cos(x-7π/4）-1】-1/2【cos(x+7π/4）-1】
=1/2【cos(x-7π/4）-cos(x+7π/4）】=sin（7π/4）sinx=-sin（π/4）sinx=-2^(1/2) sinx.