How do you know if it adds up to 180 or 90 if there is just letters? -

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an angle/ angles of 180° will form a straight line, and right angles should be labeled with a little symbol near the vertex, which looks like a quarter of a square. if you click on geometry under math, and then angles : types and labeling under geometry building blocks, there's a video with the angle types. (it wouldnt let me paste the url) i suggest you watch it because the visual makes more sense then trying to explain what it looks like. that should help :)