Angles on a straight line, step by step

Angles sitting side by side along a straight line share what is left of a half turn: together they fill 180°.

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The question

degrees

Angles on a line: x, 45°, 90°

45° + 90° + x=180°

45° + 90°=135°

180° − 135°=45°

The angles on a straight line add up to 180°, so all three together make 180°.
Add the two you are given first: 45 + 90 = 135.
Then take that from 180°: 180 − 135 = 45. So x = 45°.

How it works.

01
The angles on a straight line add up to 180°, so all three together make 180°.
02
Add the two you are given first: 45 + 90 = 135.
03
Then take that from 180°: 180 − 135 = 45. So x = 45°.