Question P6 Overlap and cancel
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Mrs Tan pins three identical equilateral triangular bunting flags, Flag 1, Flag 2 and Flag 3, along a straight horizontal string. Each flag points upwards and its bottom edge lies flat on the string. Flag 1 is on the left, Flag 2 is in the middle and Flag 3 is on the right. Flag 2 overlaps Flag 1 and also overlaps Flag 3, but Flag 1 and Flag 3 do not touch each other. On the string:
The shaded parts are the part of Flag 1 not covered by Flag 2 and the part of Flag 3 not covered by Flag 2.
Answer
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Two shapes share a region, and the shared part cancels out.
In this one. Put every base corner on one number line, and read an overlap as “side length minus shift”.
Also uses: Gap and difference
Shapes are joined, stacked, or have a piece cut off, and the question compares a perimeter or length before and after, rather than asking for an area from scratch.
Children assume perimeter changes the same way area does. Joining shapes or cutting off a corner can leave the perimeter unchanged, or change it by a different amount from the area.
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