Explicaê

01

(FUVEST 2016 1° FASE) No quadrilátero plano ABCD, os ângulos AB^\widehat{B}C e AD^\widehat{D}C são retos, AB = AD = 1, BC = CD = 2  e BD\overline{BD} é uma diagonal.

O cosseno do ângulo BC^\widehat{C}D vale

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