## Elementary Geometry for College Students (6th Edition)

$\angle$ADE=$\frac{1}{2}$$\angleD \angleA+\angleD=180 \angleA+\angleADE+\angleAED=180 \angleA+\frac{1}{2}$$\angle$D+$\angle$AED=180 $\angle$A+$\frac{1}{2}$$\angleD+\angleAED=\angleA+\angleD \angleAED=\frac{1}{2}$$\angle$D Since two angles have a measure equal to half of angle D, it must be an equilateral triangle.