Question

If tan (x) = 24/19 (in Quadrant 1), find

Answer 1

If x is in quadrant I, then both sin(x) and cos(x) are positive. The angle x/2 also belongs to quadrant I, and hence each of sin(x/2), cos(x/2), and tan(x/2) are positive.

Recall that for all x,

cos²(x) + sin²(x) = 1

and multiplying through both sides by 1/cos²(x) = sec²(x) gives another flavor of this identity,

1 + tan²(x) = sec²(x)

It follows that

sec(x) = √(1 + tan²(x)) = √937/19

which immediately gives us

cos(x) = 19/√937

and from the identity above we find

sin(x) = √(1 - cos²(x)) = 24/√937

Recall the half angle identity for cos :

cos²(x) = (1 + cos(2x))/2

which means

cos(x/2) = + √[(1 + cos(x))/2] = √[1/2 + 19/(2√937)]

Then

sin(x/2) = + √(1 - cos²(x/2)) = √[1/2 - 19/(2 √937)]

and by definition of tan,

tan(x/2) = sin(x/2) / cos(x/2) = 1/12 √[649/2 - 19 √937/2]

Just to be clear, the solutions are

$\cos\left(\dfrac x2\right) = \sqrt{\dfrac12 + \dfrac{19}{2\sqrt{937}}}$

$\sin\left(\dfrac x2\right) = \sqrt{\dfrac12 - \dfrac{19}{2\sqrt{937}}}$

$\tan\left(\dfrac x2\right) = \dfrac1{12} \sqrt{\dfrac{649 - 19\sqrt{937}}2}$