我们在文［1］～[4]中，曾给出了三角形中半角三角函数的一些不等式，本文又进一步得到了关于三角形中半角三角函数的一个不等式链，从而得到了用外接圆半径、内切圆半径以及半周长表示的三角形牛角三角函数上、下界的线性不等式的最佳表达式．本文约定：△ABC中，三边长为a、b In [1]-[4], we have given some inequalities of the trigonometric function of the triangle in the triangle. In this paper, we obtain an inequality chain of the trigonometric function of the triangle in the triangle. The best expression of the linear inequality of the upper and lower bounds of the triangular horn of the triangle horn represented by the tangent circle radius and the half circumference. This article agrees: △ ABC, the three sides are a, b