# Thread: How to get sin(x)?

1. ## How to get sin(x)?

Hi, I was solving an integral and got this substitution

t= tg(x/2) then sin(x) = (2t)/(1+t^2) and cos(x) = (1-t^2)/(1+t^2)

How did I get sin(x) = (2t)/(1+t^2)?

I know that t = sinx/(cosx+1) and sinx = [(cosx+1)*t]

2. ## Re: How to get sin(x)?

Unfortunately, you start off by saying you "know" something that isn't true!
$tan(x)= \frac{sin(x)}{cos(x)}$ so $tan(x(/2)= \frac{sin(x/2)}{cos(x/2)}= t$. We have the trig identities $sin(x/2)= \sqrt{(1/2)(1- cos(x)}$ and $cos(x/2)= \sqrt{(1/2)(1+ cos(x})}$ so $t= \sqrt{\frac{1- cos(x)}{1+ cos(x)}}$, $t^2= \frac{1- cos(x)}{1+ cos(x)}$, $t^2(1+ cos(x))= 1- cos(x)$, $(t^2+ 1)cos(x)= 1- t^2$ so that $cos(x)= \frac{1- t^2}{1+ t^2}$.

3. ## Re: How to get sin(x)?

$\tan(2A)= \frac{2\tan(A)}{1-\tan^{2}(A)}$

from which

$\tan x = \frac{2\tan(x/2)}{1-\tan^{2}(x/2)}.$

Let $t=\tan(x/2),$ and we have $\tan x = \frac{2t}{1-t^{2}}.$

Now draw a right-angled triangle with $\text{ 'base angle' } x,\text{ 'opposite' }2t\text{ and 'adjacent' }1-t^{2}.$ Use Pythagoras to calculate the hypotenuse $\text{ }1+t^{2}\text{ },$ and so read off the expressions for $\sin x \text{ and } \cos x.$

4. ## Re: How to get sin(x)?

Thank you both, I'll be studying this before my exam.