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The shaded triangle has angles of measure B, 180-A, and A - B
Thus A - B is the measure of the exterior angle of triangle XAY. Since triangle
XAY is isosceles, then the measure of the base angles is
and the measure of the acute angle at Y in triangle XYZ is
What is the length of XY? Each half of the isosceles triangle XAY is
a right triangle with hypotenuse of length 1. So each half of XY is
or,
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