If we are given the value of α. How can we find the value of β?

Answer:
β = 140 - α
we know that the "missing" angle is 40° since 40 + 140 = 180
we also know that the total sum of angles in a triangle is 180
so β = 180 - 40 - α
or β = 140 - α
Step-by-step explanation: