Find Angle B in Quadrilateral ABCD: Using 140° and 80° Measurements

Question

ABCD is a quadrilateral.

According to the data, calculate the size of $∢B$ .

00:00 Find angle B

00:09 The sum of angles in a quadrilateral is always equal to 360

00:15 Substitute appropriate values according to the given data

00:35 Collect terms and isolate D

00:55 And this is the solution to the question

As we know, the sum of the angles in a square is equal to 360 degrees, therefore:

$360=A+B+C+D$

We replace the data we have in the previous formula:

$360=140+B+80+90$

$360=310+B$

Rearrange the sides and use the appropriate sign:

$360-310=B$

$50=B$