tan(a/2)*tan[(a－b）/2]

　　＝{sin(a/2)/cos(a/2)}×{sin[(a－b）/2]/cos[(a－b）/2]}

　　＝{sin(a/2)×sin[(a－b）/2]}/{cos(a/2)×cos[(a－b）/2]}

　　＝(-1/2){cos[(a/2)+(a－b）/2]-cos[(a/2)-(a－b）/2]}/(1/2){cos[(a/2)+(a－b）/2]+cos[(a/2)-(a－b）/2]}

　　＝-{cos[a-(b/2)]-cos(b/2)}/{cos[a-(b/2)]+cos(b/2)}

　　(提示：根据tanx=sinx/cosx,然后将分子/分母积化和差）

　　∵5cos[a－(b/2)]+7cos(b/2)＝0

　　∴cos[a－(b/2)]＝(-7/5)cos(b/2)

　　∴tan(a/2)*tan[(a－b）/2]

　　＝-{cos[a-(b/2)]-cos(b/2)}/{cos[a-(b/2)]+cos(b/2)}

　　＝-{(-7/5)cos(b/2)-cos(b/2)}/{(-7/5)cos(b/2)+cos

　　(b/2)}

　　＝-[(-12/5)cos(b/2)]/[(-2/5)cos(b/2)]

　　＝(12/5)/(-2/5)

　　＝-6