To classify the triangle by its sides, we need to check the triangle inequality theorem. The theorem states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side.
In this case, we have two sides of lengths 1 cm and 13 cm. Let's denote the third side as \( x \) cm.
- \( 1 + 13 > x \) → \( 14 > x \) (So \( x < 14 \))
- \( 1 + x > 13 \) → \( x > 12 \)
- \( 13 + x > 1 \) → \( x > -12 \) (This inequality is always satisfied for positive lengths.)
To satisfy both relevant inequalities \( x < 14 \) and \( x > 12 \), the only possible whole number value for \( x \) is 13 cm.
Thus we have the side lengths as 1 cm, 13 cm, and 13 cm.
Now to classify the triangle:
- Since two sides are equal (13 cm and 13 cm), the triangle is classified as isosceles.
So the correct response is isosceles.