Reasoning · Grade 5-2 Exploring the Cube

Problem

Top, front, and side views of drawn lines

Lines are drawn on the surface of a see-through cube. It is viewed straight from above, from the front and from the right. Being transparent, lines at the back show too. Draw what each of the three views shows.
(1) top front side top front side (2) top front side top front side
Your answer
How to solve
Strategy Visualize Spatial Relationships — Trying to picture a whole triangle of slanted lines at once is what makes this feel hard, so instead I take the red lines one segment at a time and, for each segment, ask only where its two end points go. A corner of the cube always lands on a corner of the square, and the midpoint of an edge always lands on the midpoint of a side, so each segment's image is settled by two easy marks. Working through the segments in a fixed order — three of them in (1), four in (2) — and filling in a small table of end points keeps me from losing one. If it still feels slippery, a clear plastic box with elastic bands stretched along the lines, held up at eye level, shows the same three pictures directly.
1STEP 1

Give every corner an address

Address each corner by left-right, front-back, top-bottom.

corner = (left/right, front/back, top/bottom) ⟶ a view drops one of the three
2STEP 2

Fix how each empty square is oriented

Fix how each empty square is oriented.

3STEP 3

Part (1): follow the first segment, the diagonal of the top face

Take one segment and drop one address part per view.

4STEP 4

Part (1): follow the other two segments the same way

Follow the other segments the same way.

5STEP 5

Part (1): put the three lists together into three drawings

Together they complete part (1).

3 segments × 3 views = 9 images: 3 diagonals + 6 sides
6STEP 6

Part (2): the two upright segments disappear from the top view

In (2) upright segments become points from above.

upright segment seen from directly above ⟶ one point
7STEP 7

Part (2): the front view keeps only the upright middle line

From the front only the upright middle line remains.

8STEP 8

Part (2): the side view is just the square

From the side it is a plain square.

4 segments ⟶ 4 sides of the square, 0 lines inside
9STEP 9

Check by turning a real box

Turning a real box confirms all three views.

Answer
the picture with the viewing direction dropped
Every view has to be a square of exactly the same size as a face of the cube, and every answer is, so no picture has changed scale. Each red segment must show up in all three views, since a segment can vanish only in the one view taken along its own direction: in (1) all three segments appear three times each, and in (2) the two upright segments appear as points in the top view but as full sides in the other two, which is exactly the expected pattern. The end points also check out: corners of the cube always landed on corners of the square and the two edge midpoints always landed on midpoints of a side, never anywhere in between. Finally, the four segments of (2) form a closed loop on the cube, and their images do close up as well — a loop cannot open out just because you look at it from somewhere else.
Takeaway

Looking at a see-through cube from one direction just forgets one of the three directions, so lines pointing straight at you become dots and lines on a face you see edge-on hide in the square's own border!

  • Give every corner an address
  • Fix how each empty square is oriented
  • Part (1): follow the first segment, the diagonal of the top face
  • Part (1): follow the other two segments the same way
  • Part (1): put the three lists together into three drawings
  • Part (2): the two upright segments disappear from the top view
  • Part (2): the front view keeps only the upright middle line
  • Part (2): the side view is just the square
  • Check by turning a real box