Straight lines OB and CD are parallel, and OA and CD are crossing lines. Find the angle between

Straight lines OB and CD are parallel, and OA and CD are crossing lines. Find the angle between straight lines OA and CD, if: a) ∠AOB = 40 °; b) ∠АВ = 135 °; c) ∠АВ = 90 °.

Since the straight lines OB and CD are parallel, the angles at the intersection of the straight line CD and the straight line OA will injure the corners at the intersection of the straight lines OB and OA.

Then the angle AOB is equal to the angle ACD:

1) ∠ACD = ∠AOB = 40 degrees;

2) ∠ACD = ∠AOB = 135 degrees;

3) ∠ACD = ∠AOB = 90 degrees.