Two sides of a triangle measure 8 cm and 15 cm. Which could be the length of the third side?

For three lengths to form a triangle, the third side must be greater than the difference of the other two sides and less than their sum. Here, that means 15 − 8 < x < 15 + 8, or 7 < x < 23. Of the choices, 18 cm fits this range, so the third side could be 18 cm.

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