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## Main Question or Discussion Point

I think we can say that a straight line is a triangle in which sum of two side equals third side in which two angle equals 0° and the middle angle equals 180°, but can we say that a striaght line is a triangle with two angle equals 90°, I am saying this because my sister is not beliving that sin(90°) = 1, she is saying that for proving sin(90°) = 1, I am making the triangle as a straight line but these ratios are only applicable for triangles , I told her that there will be no triangle with two angle equals 90° therefore I have to make it as a straight line.