The quintessential Idli – a mathematical wonder!

A quick stroll in the early mornings, on the roads of various cities of Tamilnadu, can give you a wonderful sight of the paradigmatic idli under mass production in the road side Tiffin centres. The white steam emanating out of the ellipsoidical fermented dough of de-husked black lentils which are steamed in the shallow parabolic molds, serves as the staple tiffin the southern parts of ‘United States of India’.

The wolfram mathematics community proudly affirms the universal fact that idli and ellipsoid has fundamental Cartesian relationship with each other than any other shapes in the world of mathematics. The morning hunger can never be satisfied by any other shapes like stuffed semi spheres (Burgers) or even cylinders (Kola Puttu, No offence meant as we already have water issues with them!) which can have adverse effects on our bowel systems. The magnanimity of such oral daintiness is its cheap and easy availability and you never need a management graduate from IIM A or even a B, to come up with JIT management systems or Lean manufacturing techniques. An over-production in the idli, never ends up as a waste. It can always be re-engineered and value added to invent a new item called ‘Idli Uppuma’ or it can also be cut into pieces and can be sautéed to be christened as ‘Fried Idlis’ which can be served with same set of condiments.

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Sometime back, I was chatting with this girl ‘SHE’. The name is not revealed as it may lead to identity revelation and unwanted interrogation and expectation and assumptions from fellow mates.  I’ll tell you how this conversation started and ended in knowledge exchange and sharing which is the most desirable event in a conversation.

I was busy with the documentation work for my code testing. Suddenly a new mail alert popped out (Outlook Users understand this better) in my desktop. The Subject read – Solve this if u r genius. I opened the mail and found this one below

The mail asked me to find why there was a gap in the second triangle if the area of both the triangles, formed by rearranging the smaller pieces is same.

Since I was an engineering graduate and to the fact that, we had “Engineering Mathematics”  for first two years with one maths paper for each semester, I looked into the figure more carefully, so as to dust my brains to check any traces of mathematics data existence which I had to study and work out for two years of my engineering life.

Whoa !! Some trigonometry question!! Hmmm….Not bad, my brain still works and it is active as before. !

I quickly remembered that the area of the triangle can be calculated by adding the area of the squares (Partially filled and fully filled. Filled with colors in this context). And found few squares which are not occupied by the triangle, so I counted the number of partial squares and found that they are same. I thought the area occupied by the triangle in the partial squares in both the figures may be different.

I pinged her…

ME: u know that ans?

SHE: No, u know?

ME: Yea 🙂

SHE: Tell…

ME: The area covered by the partially filled squares

along the hypotenuse is not same in both the figures.

So I think all those differences come into Single Square

when u re-arrange the pieces

SHE: mmm…do u think the hypotenuse line cutting the squares

are slightly different points in both the images?

ME: Yea 🙂

SHE: But, between two points there can be only

one straight line.

(Oh Yes, That’s true, we cannot have more than one straight line in between two different points. Point noted !! So how can this be. I started thinking again)

ME: Yea, but u see the slopes are different in both the

cases as the angle of inclination of the hypotenuse to the

base is different.

SHE: But how? They both are right triangles,

ME: The blue and red triangle forms the

Hypotenuse rite??

SHE: yea. . . .both of them have hypotenuse forming the same line

. . . parallel sides. . both base and height so. . it should the same angle

of elevation…then. . how can it have diff slopes?

ME: Haan par the angle of inclination differs as the

length of the hypotenuse differs…

SHE: but. . still. .as length of hypotenuse differs. .

but proportionally the other two sides too…rt?

ME: u have to construct a cardboard model and see

…u’ll see that the long hypotenuse is just bent a bit…..

if u r not sure….

SHE: ok. . but.. how is it theoretically possible. .

The base lines of the triangles in both figures are parallel,

the hypotenuse are parallel. .which means. . They should have

same angle of inclination / slope rt?

ME: mmm….it is optical illusion may b….

see this link

SHE: hmmm…

ME: thats wat it says…i told u na….the hypotenuse

lines are different in both cases… says in a

different way…it says it forms a quadrilateral

SHE: for the quadrilateral. . its understandable,

because. . The length of the sides gets increase,

so. .it was obvious dat there should be a gap somewhere

….for here, the base length and height are still the same and

if u look at the first diagram, which is sort of animated they flip

the blue and red triangles, the blue one exactly fits into the red one

’cause they are proportional. . dat is…of course only a visual thing. .

actual measurements can differ..

But. . I don’t see a reason y they shouldn’t have the same

slope / angle of slope

ME: hmmm…yea…but visually they appear proportional…

but wen u chk with a card board or chart paper model

u’ll know !! even I’m not satisfied with the wiki thing…

only wen u chk practically u will know..

SHE: OK.. I’m getting it. .. but . . .

y wont it have same slope????

ME: Equation of straight line, y-y1 = m(x – x1)

m = (y – y1)/x – x1)

‘m’ is slope…according to my knowledge, since the difference

between x and x1 & y and y1 differs, the slope differs..

SHE: :S…dunno….will have to actually calculate

ME: I did

for red,  m = 3/8
for blue, m = 2/5

it differs very slightly…

SHE: haan

0.375 and 0.4


ME: yup

Exactly….so the combined hypotenuse line doesn’t cut the

squares at same points in both the figures.

doubt cleared uh?

SHE: 😀



ME: hehe…!!

now i got a gud topic to blog !!


SHE: blog?

ME: I’ll put the conv in blog with some edit !!

SHE: How??

On what??

ME: disguise the names

SHE: u r gonna blog about a missing square??

ME: u havent seen chat blogs??

SHE : ???

good good

nope. . not much

this will be tirst one i guess

when u write and post

ME: hey u wud have seen that

lots of ppl rite

SHE: not really

I have seen blogs where ppl mention a few lines from a chat

and write the rest on it

or mention the line somewhere in the chat


my frnd told me. . .etc

ME: oh…

anyways..see wen i post

SHE: hehe…all right


In the end of the conversation, I could conclude that the slope thing was there and the hypotenuse is not cutting the partially filled squares at same points in both the figures. So the remaining area of the partially filled squares along the hypotenuse of figure 1, comes as a single unfilled square in the figure two when the shapes are rearranged!

So with this, conversation ended for a few minutes. I really do not know if we have come to a conclusion about this missing square. Or this is still an unsolved one. But both of us were satisfied that we found an answer for the missing square and we were able to justify our conclusions with the mathematical proof.

Out conclusions may be wrong….If u have any other answers, please post it in the comments!!!