tan（50°-20°）=tan50°-tan20°/（1+tan50°·tan20°）=tan30°=根号3/3
得tan50°-tan20°=根号3/3+（根号3/3）·tan50°·tan20°
代入上述tan50°-tan20°-(根号3/3)·tan50°·tan20°
=根号3/3+（根号3/3）·tan50°·tan20°-(根号3/3)·tan50°·tan20°
=根号3/3