Intr-un paralelogram unghiurile opuse sunt congruente, iar cele alaturate, suplementare. Deci:
m(∡D)=m(∡B)
m(∡A)=m(∡C)=180-m(∡B).
a)m(∡D)=m(∡B)=40
m(∡A)=m(∡C)=180-m(∡B)=180-40=140.
b)m(∡D)=m(∡B)=50
m(∡A)=m(∡C)=180-m(∡B)=180-50=130.
c)m(∡D)=m(∡B)=100
m(∡A)=m(∡C)=180-m(∡B)=180-100=80
d)m(∡D)=m(∡B)=70*30'
m(∡A)=m(∡C)=180-m(∡B)=180-70*30'=109*30'.