Question

在三角形 $CAT$ 中， $\angle ACT =\angle ATC$ ，且 $\angle CAT = 36^\circ$ 。如果 $\overline{TR}$ 平分 $\angle ATC$ ，那么 $\angle CRT =$

In triangle $CAT,$ we have $\angle ACT =\angle ATC$ and $\angle CAT = 36^\circ.$ If $\overline{TR}$ bisects $\angle ATC,$ then $\angle CRT =$

$16^\circ$

$51^\circ$

$72^\circ$

$90^\circ$

$108^\circ$

答案：C

知识点：导角角平分线

难度评级：1220

解答：

我们有 $\begin{aligned} \angle CAT + \angle ATC \\ \quad {}+ \angle ACT &= 180 \\ 36 + 2 \cdot \angle ATC &= 180 \\ \angle ATC &= 72^{\circ}. \end{aligned}$

因为该角被平分，所以 $\angle RTC = 72 \div 2 = 36^{\circ}.$

最后， $\begin{aligned} \angle RTC + \angle TCR \\ \quad {}+ \angle CRT &= 180 \\ 36 + 72 + \angle CRT &= 180 \\ \angle CRT &= 72^{\circ}. \end{aligned}$

所以正确答案是 C。

We get $\begin{aligned} \angle CAT + \angle ATC \\ \quad {}+ \angle ACT &= 180 \\ 36 + 2 \cdot \angle ATC &= 180 \\ \angle ATC &= 72^{\circ}. \end{aligned}$

Due to bisection, we also know that $\angle RTC = 72 \div 2 = 36^{\circ}.$

Finally, we see that $\begin{aligned} \angle RTC + \angle TCR \\ \quad {}+ \angle CRT &= 180 \\ 36 + 72 + \angle CRT &= 180 \\ \angle CRT &= 72^{\circ}. \end{aligned}$

Thus, C is the correct answer.

2000 AMC 8 第 13 题

答案：C

难度评级：1220

解答：

其他年份的第 13 题