24
$\begingroup$

This question already has an answer here:

enter image description here

Can anyone tell me please where does the gap come from?

Thanks and sorry if the question is not exactly relevant, I just didn't know where else to ask.

$\endgroup$

marked as duplicate by Henry, S.C.B., user91500, gebruiker, Macavity Apr 28 '16 at 10:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 11
    $\begingroup$ The slope angle of the red and blue pieces is different. $\endgroup$ – almagest Apr 26 '16 at 19:12
  • 7
    $\begingroup$ Or, watch carefully the points $(5,2)$ and $(8,3)$ on both pictures. $\endgroup$ – Berci Apr 26 '16 at 19:27
  • 1
    $\begingroup$ Martin Gardner has an informative chapter on this in his book Mathematics, Magic, and Mystery. $\endgroup$ – Brian Tung Apr 27 '16 at 4:26
  • $\begingroup$ This question gave me a huge craving for infinite chocolate! $\endgroup$ – heltonbiker Apr 27 '16 at 18:38
  • 1
    $\begingroup$ Some people would say this is an optical illusion. Your brain seeing what it expects to see. I say take a lesson from wood shop, pick up the paper, and sight down the lines. You'll see they aren't really straight without doing any math. $\endgroup$ – candied_orange Apr 27 '16 at 20:35
27
$\begingroup$

Figure out the area of the little gap in the middle:

enter image description here

(you can probably guess it, but it's easy to calculate).

Try it for yourself before peeking at the explanation at the bottom.

From there it's easy to see where the area for the square comes from.


Total area of the 13 × 5 region = 65 squares.

Total area of the two unmarked rectangular parts = (5 × 3) × 2 = 30

Total area of blue triangles = (5 × 2 × ½) × 2 = 10

Total area of red triangles = (8 × 3 × ½) × 2 = 24

Remaining area = 65 - 30 - 10 - 24 = 1 square.

If it's still not clear, rotate the upper two triangles each about the center of its own diagonal (which leaves the upper edge of the gap unchanged), you get the triangles from your two diagrams overlaid on each other in the correct positions:

enter image description here

It takes the extra square to make up for the area of that skinny parallelogram-shaped gap between the regions covered by the two arrangements of shapes.

$\endgroup$
56
$\begingroup$

If I did this

enter image description here

Would you ask where the hole came from?

Addeudum

enter image description here

$\endgroup$
  • 10
    $\begingroup$ Yes, actually I probably would... to my eyes that hole looks way more massive than the added area! $\endgroup$ – Mehrdad Apr 27 '16 at 9:10
  • 1
    $\begingroup$ @Mehrdad - Than what added area? All I did was move four objects around. The only difference is that I exagerrated the dimensions of the two triangles. $\endgroup$ – steven gregory Apr 27 '16 at 10:25
  • 1
    $\begingroup$ @StevenGregory In the lower figure, you also added a line creating a new white rectangle, and thus increased (compared to the upper figure) the area surrounded by lines. $\endgroup$ – JiK Apr 27 '16 at 10:35
  • 1
    $\begingroup$ @StevenGregory: No, I'm just looking at the overall shape, not the individual shapes. I'm saying the difference of the two shapes' areas seems to be way less than that of the white rectangle. $\endgroup$ – Mehrdad Apr 27 '16 at 11:25
  • 1
    $\begingroup$ @Mehrdad - I added another picture. Does that help? Note that the difference in area will be 1 if, for example, b,h,B,H are consecutive Fibonacci numbers. $\endgroup$ – steven gregory Apr 28 '16 at 16:29
25
$\begingroup$

The line over the red part is not quite as steep as that over the blue part. Just compute the two slopes and you'll see that.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.