Draw the perimeter of a triangle with one corner equal to 100 °. Measure one of the sharp corners.

∠BAC = 100 °; 100> 90, therefore ∠BAC is dull; the sum of the angles of a triangle is 180 °, so any of the remaining angles is sharp (180-100 = 80, and 80 <90). We measure ∠ABC:
∠ABC = 50 °. 50 <90, so the angle is sharp.

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