½ ×７× 18 = 63cm² (Abs)
【assumed ∠ACB = 90°】

Thank u

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You can try to see why the formula ½ × base × height always works for any triangle (whether right-angled, acute or obtuse)


Base of shaded triangle = 21 cm - 7 cm = 14 cm


Area of the big triangle = ½ × 21 cm × 18 cm

Area of the shaded triangle = ½ × 14 cm × 18 cm

Area of unshaded triangle = Area of big triangle - area of shaded triangle

= ½ × 21 cm × 18 cm - ½ × 14 cm × 18 cm

= ½ × (21 - 14)cm × 18 cm

= ½ × 7 cm × 18 cm

= 7 cm × 9 cm

= 63 cm²


This is the same as doing ½ × 7 cm (base) × 18 cm (height) directly

Thanks J