Question

if m∠acd = 40°, then m∠dcb = ?°

Explanation:

Step1: Recall linear - pair property

$\angle ACD$ and $\angle DCB$ form a linear pair. The sum of angles in a linear pair is $180^{\circ}$. So, $m\angle ACD + m\angle DCB=180^{\circ}$.

Step2: Solve for $m\angle DCB$

Given $m\angle ACD = 40^{\circ}$, we substitute into the equation: $m\angle DCB=180^{\circ}-m\angle ACD$. Then $m\angle DCB = 180 - 40$.

Answer:

$140$