Question 1200226
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Triangle inequalities say that the third side of the triangle is shorter than 
the sum of the two other sides

    third side is shorter than 9 + 12 = 21 cm.


The same inequalities say that the third side of the triangle is longer than 
the difference of the two other sides

    third side is longer than 12 - 9 = 3 cm.


It leaves possible options 4 cm, 10 cm and 18 cm 

and excludes other options for the length of the third side.
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