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Try this beautiful problem from Singapore Mathematics Olympiad, SMO, 2008 based on Trigonometry.

## Problem on Trigonometry | SMO, 2008 |Problem 22

Find the value of $\frac {tan 40^\circ tan 60^\circ tan 80^\circ}{tan40^\circ + tan 60^\circ + tan 80^\circ}$

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### Key Concepts

Trigonometry

Tan Rule

But try the problem first…

Source

Challenges and Thrills – Pre College Mathematics

## Try with Hints

First hint

If you got stuck in this sum how to get started you can start by consider a general case where $40^\circ = A$ , $60^\circ = B$ and $80^\circ = C$.

So , A+B+C = $180 ^\circ$

$A+B = 180^\circ – C$

(tan (A+B) = tan $180^\circ – C)$……………………….(1)

Now try to implement the basic formula and try to do this sum………………

Second Hint

In this we can continue from the last hint:

the formula of tan (A + B) = $\frac {tan A + tan B}{1- tan A . tan B}$

From the equation (1) …….

tan (A+B) = tan (180 – C)

$\frac {tan A + tan B}{1- tan A . tan B} = tan (180^\circ – c)$

(frac {tan A + tan B}{1- tan A . tan B} = -tan C )

Now just rearrange this expression and you will get the final answer……………..

Final Step

Here is the final solution:

{tan A + tan B} = -tan C {1- tan A . tan B}

tan A + tan B = – tan C + tan C tan A tan B

tan A + tan B + tan C = tan A tan B tan C

$\frac {tan A tan B tan C}{tan A + tan B + tan C} = 1$

Which is the given question. It can be a proof also……………….