https://www.dailydot.com/upstream/wolfie6020-flat-earth-challenge-youtube
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A triangle with 3 right angles is not possible on a 2D flat plane. However, it is possible on the surface of a sphere or 3D ellipsoid.

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Spherical triangles are one of the many proofs of the spheroidial earth.
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The simplest example is to begin at one of the poles, travel toward the opposite pole in a straight course.
     Upon reaching the equator (traversing 90 degrees), turn 90 degrees right or left and proceed due east (or west) over 90 degrees.
     Then turn the same way again, and proceed 90 degrees a second time, to find you have returned to the starting point.
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Impossible on any "flat" earth. 

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It's nice Wolfie6020 (who does not monetize his channel) made these videos so we can see what's going on.
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Because if we relied on the flat-earthdom syndromers to provide the goods we would have no idea what's happening.
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This clown, flat out, has no job and only scams gullible flat-earthdom syndromers instead of working for his living. What a fake.
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He monetizes his videos and collects money from sheeple who watch them by the ad revenue, but he lies through his teeth.
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The challenge he made was met, then he changed his requirements; the new ones were met and he replied by changing the game again.
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So his subscriber base has shrunk to almost nothing, and the negative comments under his videos are so numerous he can't keep up with deleting them.
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What a joke.          -------------->>> Get used to it, folks, there will be many more fiascoes like this one.