Verified answer

(5*корень(2))^2=5^2+4^2-2*5*4*cos(alpha)

(x)^2=5^2+12^2-2*5*12*cos(alpha)

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cos(alpha)=(5^2+4^2-(5*корень(2))^2)/(2*5*4)

(x)^2=5^2+12^2-2*5*12*cos(alpha)=

=5^2+12^2-2*5*12*(5^2+4^2-(5*корень(2))^2)/(2*5*4)=

=5^2+12^2-3*(5^2+4^2-(5*корень(2))^2)=

=196

x=корень(196)=14