Answer :

[tex]The\ largest\ angle\ is\ opposite\ the\ largest\ side.\\look\ at\ the\ picture.\\\\Law\ of\ cosines:\\\\c^2=a^2+b^2-2abcos\alpha\\\\a=3;\ b=5;\ c=7\\\\substitute:[/tex]

[tex]7^2=3^2+5^2-2\cdot3\cdot5\cdot cos\alpha\\49=9+25-30cos\alpha\\49=34-30cos\alpha\\30cos\alpha=34-49\\30cos\alpha=-15\ \ \ \ /:30\\cos\alpha=-\frac{15}{30}\\\\cos\alpha=-\frac{1}{2}\\\\\alpha=120^o[/tex]
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