解法1：套用公式：cosα·cosβ=(1/2)[cos(α+β)+cos(α-β)]有：cos(π/4-a)cos(π/4+a)=(1/2)[cos(π/4-a+π/4+a)cos(π/4-a-π/4-a)]=(1/2)[cos(π/2)+cos(-2a)]=(1/2)[0+cos(2a)]=(1/2)cos(2a)解法2：cos(π/4-...