Answer
m$\angle$D=221$^{\circ}$
Work Step by Step
m$\angle$A+m$\angle$B+m$\angle$C+m$\angle$D=360$^{\circ}$
20+88+31+m$\angle$D=360
m$\angle$D=221$^{\circ}$
Elementary Geometry for College Students (5th Edition)
by Alexander, Daniel C.; Koeberlein, Geralyn M.
Published by
Brooks Cole
ISBN 10:
1439047901
ISBN 13:
978-1-43904-790-3
Chapter 2 - Section 2.5 - Convex Polygons - Exercises - Page 107: 41
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