АК^2=5^2+6^2=25+36=61
AK=sqrt(61)
AC_(1)=sqrt(5^2+3^2+4^2)=sqrt(50)
C_(1)K=5
По теореме косинусов
AK^2=AC^2_(1)+C_(1)K^2-2*AC_(1)*C_(1)K*cos ∠ AC_(1)K
61=50+25-2*sqrt(50)*5*cos ∠ AC_(1)K ⇒
cos ∠ AC_(1)K=(75-61)/(10*sqrt (50))=7/(5*sqrt(2))