Yesterday my sir asked us a question:"How can you find the value of tan2° without using the calculator? " I asked, whether he is asking the formula of tan 2A or something, but he said no its tan 2°. I tried my head out in every possible way even tried out the approximation method of differentiation, but didn't got any idea. May be it will be something like tan (60°/30°) or something like that, but I get no clue. The exact value is 0.035, but that's coming from a calculator. How to find ourselves the value? Any idea? And I'm not familiar with MathJack so would be grateful if someone edit it out for me. Thanks in advance!

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    $\begingroup$ For small $x$, $\tan{x} \approx x$, when $x$ is in radians. So $\tan{2}= \approx \frac{2 \pi}{180}$. $\endgroup$
    – Paul
    Aug 30 '16 at 16:14
  • $\begingroup$ $\tan 2^\circ$ is not exactly 0.035. That's an approximation. $\endgroup$
    – user307169
    Aug 30 '16 at 16:21
  • $\begingroup$ @Paul yup I got it.... $0.0175*2=0.035$..that's the approx value....Thanks! $\endgroup$
    – Aneek
    Aug 30 '16 at 16:25
  • $\begingroup$ @Paul Why not write it up as an answer? I'll upvote your answer. $\endgroup$
    – amWhy
    Aug 30 '16 at 16:35
  • $\begingroup$ @amWhy May I write? I hope Paul hasn't started working on it...waiting for response from Paul $\endgroup$
    – Aneek
    Aug 30 '16 at 16:36

Okay so it seems I have found it...

I hope all know for very small angles $\sin\theta$ and $\tan\theta$ become nearly equal to $\theta$, in radians. I hope I don't need to prove that.

So since $2^\circ=2\cdot 0.0175$ radians, or $2^\circ=\frac {2\pi}{180}$,

we get that $\tan(2^{\circ})$ is approximately equal to $0.0349.$

  • $\begingroup$ "I hope I don't need to prove that" math.stackexchange.com/questions/448207/… $\endgroup$
    – leonbloy
    Aug 30 '16 at 16:55
  • $\begingroup$ Nice work finding that, @leonboy! $\endgroup$
    – amWhy
    Aug 30 '16 at 16:56
  • $\begingroup$ @leonbloy you made my answer more complete! Thanks! $\endgroup$
    – Aneek
    Aug 30 '16 at 17:01

The linear approximation formula says that $f(x+\Delta x)\approx f(x)+f'(x)\Delta x$

Let $y=f(x)=\tan x$. Set $x=0$ and $\Delta x=2^{\circ}=\frac{\pi}{90}$ radians.

$\therefore f(0+2^\circ)\approx f(0)+f'(0)\frac{\pi}{90}$

$\implies f(2^\circ)\approx \tan 0+\sec^20\times\frac{\pi}{90}$

i.e., $\tan 2^\circ\approx\frac{\pi}{90}\approx 0.0349.$

Note that we can exclude the approximation of $\frac{\pi}{90}$ which would require a calculator.

  • $\begingroup$ Nice work...I was thinking of using approximating formula to calculate it, and you came up with it...well done! And yes, if you know the conversion ratio of degree to radian then I guess you don't have to use the calculator at all! $\endgroup$
    – Aneek
    Sep 4 '16 at 10:24
  • $\begingroup$ How is 2 degrees = $2\pi/180$ ? One measure in radian, another is in degrees. $\endgroup$
    – user342531
    Aug 2 '17 at 6:58
  • $\begingroup$ @Abcd That's the well known conversion 'formula' so to speak. Did you check out this article on Wikipedia? $\endgroup$ Aug 3 '17 at 15:37

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