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Supplementary angles, step by step

Two supplementary angles sit along a straight line, so they add to 180 degrees.

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The question
25°x degrees

Angles on a straight line, one is 25°

25°+x=180°
180 − 25 =155°
25°+155°=180°
from the picture, not the numbers
lines crossing:x = the angle opposite
  1. The two angles sit on a STRAIGHT LINE. A straight line is a half turn, which is 180 — not 90, and the picture is the only thing that tells you which.
  2. So the two must add to 180: x = 180 − 25 = 155°.
  3. If it had been the other kind you would have taken it from 90 instead and been 90° out — which is why the picture is read first.
  4. One more shape to know: where two straight lines CROSS, the two angles facing each other are equal — nothing is taken away at all. Subtracting there would be wrong.

How it works.

  1. 01

    The two angles sit on a STRAIGHT LINE. A straight line is a half turn, which is 180 — not 90, and the picture is the only thing that tells you which.

  2. 02

    So the two must add to 180: x = 180 − 25 = 155°.

  3. 03

    If it had been the other kind you would have taken it from 90 instead and been 90° out — which is why the picture is read first.

  4. 04

    One more shape to know: where two straight lines CROSS, the two angles facing each other are equal — nothing is taken away at all. Subtracting there would be wrong.

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