Question

ABCD RECTANGLE α + β = ?

Answer 1

α + β = 130

Explanation:

∠ A = ∠ C = 90°

The sum of the 3 angles in a triangle = 180°

vertex angle at D inside the Δ = 180 - (90 + α ) ← Δ on left

vertex angle at D inside the Δ = 180 - (90 + β ) ← Δ on right

∠ ADC = 90° thus

180 - (90 + α) + 180 - (90 + β) + 40 = 90

180 - 90 - α + 180 - 90 - β + 40 = 90, that is

220 - α - β = 90 (add α and β to both sides )

220 = 90 + α + β (subtract 90 from both sides )

130 = α + β

Answer 2

Explanation:

I'm going to walk through this analytically, so I will have to assign some variables to angles that are not marked. Pay close attention so you can follow the logic.

The angle at the top left next to and to the left of 40 will be "x", and the one to the right of 40 will be "y". Because that angle is a right angle, then we know that

x + y + 40 = 90 and

x + y = 50.

We also know that, by the Triangle Angle-Sum Theorem, the 2 triangles that contain alpha and beta will add up to equal 360, 180 apiece. So now we have:

x + 90 + α + y + 90 + β = 360.

Let's regroup a bit:

x + y + α + β + 90 + 90 = 360 and

(x + y) + α + β + 180 = 360.

But we know from above that x + y = 50, so

50 + α + β + 180 = 360 and

230 + α + β = 360 and

α + β = 130. There you go!