# The t Method

“The “t” Method”

Hey
Recently I have been studying for an upcoming test where it requires me to use “The “t” Method”. In this method the value of x for trigonometric equations is determined through vair the key component of “The “t” Method” is:
$$t=tan \frac{A}{2}$$
$$tan A=\frac{1+t^2}{1-t^2}$$
$$sin A=\frac{2t}{1+t^2}$$
$$cos A=\frac{1-t^2}{1+t^2}$$
If this is famliar to a reader by another name, could you please post this methods other name.
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Problems which i need to solve:
1. $$2sin x + 3cos x = 5$$
2. $$3tan x + \sqrt{3}sec x=1$$
3. $$10cos (pi x) + 3sin (2pi x)=4$$
4. $$3sin 2x + 5cot 3x = 7$$
5. $$csc x + 2sec (pi x)$$
__________________________________________________​
My working for question 1. $$2sin x + 3cos x = 5$$
$$2\frac{2t}{1+t^2} + 3\frac{1-t^2}{1+t^2} = 5$$
$$\frac{4t + 3 - 3t^2}{1+t^2} = 5$$
$$4t + 3 -3t^2 = 5 +5t^2$$
$$4t + 2t^2 = 2$$
$$2t^2 + 4t - 2 = 0$$
$$t = 0.4142135624$$ or $$t = -2.414213562$$
$$t = \tan\frac{x}{2}$$
$$x = 2tan^-1 0.4142135624$$ or $$x = 2tan^-1 -2.414213562$$
$$x = 45$$ or $$x = -135.0005034$$
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I conclude my working here as I am not sure if it is correct. Is there an aleternaitve method I could use to check these final answers? And are there any more values of x for which the equation is true?

#### HallsofIvy

Hey
Recently I have been studying for an upcoming test where it requires me to use “The “t” Method”. In this method the value of x for trigonometric equations is determined through vair the key component of “The “t” Method” is:
$$t=tan \frac{A}{2}$$
$$tan A=\frac{1+t^2}{1-t^2}$$
$$sin A=\frac{2t}{1+t^2}$$
$$cos A=\frac{1-t^2}{1+t^2}$$
If this is famliar to a reader by another name, could you please post this methods other name.
__________________________________________________​
Problems which i need to solve:
1. $$2sin x + 3cos x = 5$$
2. $$3tan x + \sqrt{3}sec x=1$$
3. $$10cos (pi x) + 3sin (2pi x)=4$$
4. $$3sin 2x + 5cot 3x = 7$$
5. $$csc x + 2sec (pi x)$$
__________________________________________________​
My working for question 1. $$2sin x + 3cos x = 5$$
$$2\frac{2t}{1+t^2} + 3\frac{1-t^2}{1+t^2} = 5$$
$$\frac{4t + 3 - 3t^2}{1+t^2} = 5$$
$$4t + 3 -3t^2 = 5 +5t^2$$
$$4t + 2t^2 = 2$$
You've lost track of a sign: adding 3t2 to both sides gives $8t^2- 4t+ 2= 0$ or $4t^2- 2t+ 1= 0$
That has only complex roots.

In fact, since 2+ 3= 5 and sine and cosine are never larger than 1, the only way we could have 2sin x+ 3cos x= 5 is to have sin x= 1 and cos x=1 which is not true for any x.

thanks HallofIvy.
do they all have complex roots, or do i need to wokr them as i have done before to determine x when i have it in terms of 2tan^-1 t ?

#### HallsofIvy

Well, I don't know! I only looked at the first as that was the one for which you showed your work. I imagine that at least some of these have real solutions but you will have to do the algebra to see.

oh okay, ill post my wokring for the otherss when i have more time.

working for question number 2.

$$3tan x + \sqrt{3}sec x=1$$

$$3\frac{2t}{1-t^2} + \sqrt{3}\frac{1+t^2}{1-t^2} = 1$$
$$\frac{6t + \sqrt{3} + t^2\sqrt{3}}{1 - t^2} = 1$$
$$6t + \sqrt{3} + t^2\sqrt{3} = 1 - t^2$$
$$(1 + \sqrt{3})t^2 + 6t - (1 - \sqrt{3}) = 0$$
therefore: t = -0.1296640194 or t = -2.066488403
for t = -0.1296640194
$$t = tan^-1 \frac{x}{2}$$
$$x = 2tan^-1 -0.1296640194$$
$$x = -14.77596194$$
t = -2.066488403
$$t = tan^-1 \frac{x}{2}$$
$$x = 2tan^-1 -2.066488403$$
$$x = -128.3541404$$

again, im not sure if what i am do is right so i would appriciate it iof somebody helped me out. if anybody knows of alternative ways of solving these equations, please tell me how

many thanks

#### 0rthodontist

This is also known as the Weierstrass substitution.

#### eglipo

Hey
Recently I have been studying for an upcoming test where it requires me to use “The “t” Method”. In this method the value of x for trigonometric equations is determined through vair the key component of “The “t” Method” is:
$$t=tan \frac{A}{2}$$
$$tan A=\frac{1+t^2}{1-t^2}$$
$$sin A=\frac{2t}{1+t^2}$$
$$cos A=\frac{1-t^2}{1+t^2}$$
If this is famliar to a reader by another name, could you please post this methods other name.
__________________________________________________​
Problems which i need to solve:
1. $$2sin x + 3cos x = 5$$
2. $$3tan x + \sqrt{3}sec x=1$$
3. $$10cos (pi x) + 3sin (2pi x)=4$$
4. $$3sin 2x + 5cot 3x = 7$$
5. $$csc x + 2sec (pi x)$$
__________________________________________________​
My working for question 1. $$2sin x + 3cos x = 5$$
$$2\frac{2t}{1+t^2} + 3\frac{1-t^2}{1+t^2} = 5$$
$$\frac{4t + 3 - 3t^2}{1+t^2} = 5$$
$$4t + 3 -3t^2 = 5 +5t^2$$
$$4t + 2t^2 = 2$$
$$2t^2 + 4t - 2 = 0$$
$$t = 0.4142135624$$ or $$t = -2.414213562$$
$$t = \tan\frac{x}{2}$$
$$x = 2tan^-1 0.4142135624$$ or $$x = 2tan^-1 -2.414213562$$
$$x = 45$$ or $$x = -135.0005034$$
__________________________________________________​
I conclude my working here as I am not sure if it is correct. Is there an aleternaitve method I could use to check these final answers? And are there any more values of x for which the equation is true?
You’ve made a mistake between the third and forth lines. The moves to the left with a minus, so as a result you get , which has no real solutions use quadratic formula).

Alternative approach to trig

okay,many thanks once again for the replies

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