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ABCDA'B'C'D'-paralelipiped dreptunghic
AB=6 cm ; AD=3√2 cm ; AA'=9√2 cm
Aflati m[<(BD' , (ABC)]


Răspuns :

In planul ABCD
DB=√AD²+AB²=3√6
In planul BB'D'D
D'B=√D'D²+DB²=6√6
In acelasi plan cosB=DB/D'B=1/2
Deci <B=60°
Succes!