Question

如果 $\angle CBD$ 是直角，那么这个量角器显示 $\angle ABC$ 的度数大约是

If $\angle CBD$ is a right angle, then this protractor indicates that the measure of $\angle ABC$ is approximately

$20^\circ$

$40^\circ$

$50^\circ$

$70^\circ$

$120^\circ$

知识点：导角

从量角器读数可见，射线 $BA$ 约在 $20^\circ$ ，射线 $BD$ 约在 $160^\circ$ 。

因为 $\angle CBD$ 是直角，所以射线 $BC$ 在 $160^\circ - 90^\circ = 70^\circ$ 。

因此 $\angle ABC = 70^\circ - 20^\circ = 50^\circ$ 。

所以正确答案是 C。

Reading the protractor, ray $BA$ is at about $20^\circ$ and ray $BD$ is at about $160^\circ.$

Because $\angle CBD$ is a right angle, ray $BC$ is at $160^\circ - 90^\circ = 70^\circ.$

Therefore $\angle ABC = 70^\circ - 20^\circ = 50^\circ.$

Thus, the correct answer is C .