cos(π/4-a)=3/5 (π/4-a)∈(-π/2,0)
得到sin(π/4-a)=-4/5
sin(π/4+β)=12/13 (π/4+β)∈(π/4,π/2)
得到cos(π/4+β)=5/13
cos(a+β)=cos[(π/4+β)-(π/4-a)]=cos(π/4+β)cos(π/4-a)+sin(π/4+β)sin(π/4-a)
=(5/13)*(3/5)+(12/13)*(-4/5)
=-33/65
cos(π/4-a)=3/5 (π/4-a)∈(-π/2,0)
得到sin(π/4-a)=-4/5
sin(π/4+β)=12/13 (π/4+β)∈(π/4,π/2)
得到cos(π/4+β)=5/13
cos(a+β)=cos[(π/4+β)-(π/4-a)]=cos(π/4+β)cos(π/4-a)+sin(π/4+β)sin(π/4-a)
=(5/13)*(3/5)+(12/13)*(-4/5)
=-33/65