If \( \mathrm{ABCD} \) is a square of side length \( 6 \mathrm{~cm} \) then the area of triangle \( \mathrm{ABC}=\ldots \ldots \mathrm{cm}^{2} \)

Step 1 ：\( \triangle ABC \) is a right-angled triangle with legs \( AB \) and \( BC \) each of length \( 6 \mathrm{~cm} \)

Step 2 ：Using formula for the area of a right-angled triangle: \( A = \frac{1}{2} \times base \times height \)

Step 3 ：Substitute \( base = 6 \mathrm{~cm} \) and \( height = 6 \mathrm{~cm} \): \( A = \frac{1}{2} \times 6 \times 6 = 18 \mathrm{cm}^2 \)