By definition, 3 side lengths are a triangle if any 2 side lengths add up to be greater than the third.
We can already see that the side lengths 6 and 12 will add up to a value of 18, which is equal to the last side length, meaning that the three values are not a triangle.
[tex]a=6[/tex]
[tex]b=12[/tex]
[tex]c=18[/tex]
[tex]a+b>c[/tex]
[tex]b+c>a[/tex]
[tex]c+a>b[/tex]
[tex]6+12>18[/tex] (Since this is not true, this is not a triangle)
Hope this helps.
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