E2124

Problem E2124 from American Mathematical Monthly, Vol. 75, No. 8 (Oct. 1968), p. 899:
for any ABC, AB1C1 is an isosceles right triangle.
Proof: expressing the vectors AB1 and AC1 through u and v as suggested by the vectors drawn reveals that AC1 is the ‘perp’ (i. e. the 90° left turn) of AB1.
Drag A, B, or C to see the rest of the drawing change accordingly
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