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