Triangle ABC is a right triangle. Point D is the midpoint of side AB, and point E is the midpoint of side AC. The measure of ADE is 47. Triangle ABC with segment DE. Angle ADE measures 47 degrees. The proof, with a missing reason, proves that the measure of ECB is 43. Statement Reason mADE = 47 Given mDAE = 90 Definition of a right angle mAED = 43 ? segment DE joins the midpoints of segment AB and segment AC Given segment DE is parallel to segment BC Midsegment of a Triangle Theorem ECB AED Corresponding angles are congruent mECB = 43 Substitution property Which theorem can be used to fill in the missing reason? Concurrency of Medians Theorem Isosceles Triangle Theorem Triangle Inequality Theorem Triangle Sum Theorem