[tex]m_a= \frac{a+b+c}{3} \ \textless \ =\ \textgreater \ 21= \frac{a+b+c}{3} \\\\
m_a= \frac{a+b}{2} \ \textless \ =\ \textgreater \ 25= \frac{a+b}{2}=\ \textgreater \ a+b=25*2=\ \textgreater \ a+b=50\\\\
c=2*a-7\\
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=\ \textgreater \ 21= \frac{50+c}{3} =\ \textgreater \ 50+c=63=\ \textgreater \ c=63-50=\ \textgreater \ c=13\\\\
=\ \textgreater \ 13=2a-7=\ \textgreater \ 2a=20=\ \textgreater \ a=20:2=\ \textgreater \ a=10\\\\
25= \frac{10+b}{2} =\ \textgreater \ 10+b=50=\ \textgreater \ b=40\\\\
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Proba: \frac{40+10+13}{3} =\ \textgreater \ 63:3=\ \textgreater \ 21(corect)\\\\
\frac{40+10}{2}=50:2=\ \textgreater \ 25(corect)\\\\
13=2*10-7=\ \textgreater \ 13=20-7=\ \textgreater \ 13=13(corect)
[/tex]
