Shape and Space 4
1 mark · EasyExplain why the diagonals of a rectangle are always equal in length.
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Both diagonals are between opposite corners and form congruent triangles; the sides of the rectangle are equal in pairs, so by Pythagoras, their lengths are equal.
Also accept: Because both are found using Pythagoras' theorem with the same sides.; Diagonals join pairs of equal sides, making them equal by symmetry.; Both diagonals have the same length as they are calculated using the same two side lengths.
Common mistakes: Thinking only squares have equal diagonals.
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Marking points
B1 · 1 mark · Recognises diagonals are congruent by Pythagoras/side lengths/symmetry or congruent triangles.
Worked solution
Diagonals connect opposite corners of a rectangle. Since rectangles have equal opposite sides and right angles, both diagonals span equal distances and can be calculated using Pythagoras' theorem with the same pair of side lengths.