Video narration
On graph paper, how can you tell which of two triangles has more area when they look different? Each square is one square unit, so the grid lets you measure instead of judging by shape alone. Look at rectangle ABCD: its diagonal splits it into two equal parts. Triangle BAD is one half of that rectangle. Now follow triangle ABE, split into AEF and BEF. AEF is half of rectangle AFED; BEF is half of rectangle BFEC. Those smaller rectangles fit together to make ABCD. So the two red pieces together are half of ABCD. Different-looking triangles. Same half. Your turn: which rectangle, or pair of rectangles, helps you find another triangle's area on the grid? Keep track of the square units. Take a moment to think. On the grid, both are half of ABCD, so neither has more area.