What is the complete solution set of the equation 1+2cosec x=(-sec²x/2)/2?

Question

3098 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2018-08-12T07:49:34

$x = 2kpi- pi/2 = (4k - 1 )pi/2, k = 0, +-1, +-2, +-3, ...$ .

This angle separates $Q_3 and Q_4$ .

With $t = tan (x/2),$

$sin x = ( 2 t )/( 1 + t^2) and sec^2 ( x/2 ) = 1 + t^2$ .

Now, the given equation changes to

$1 + 2 ( (1 + t^2 )/(2 t )) +1/2( 1 + t^2 ) = 0$ , giving

$t^3 + 2t^2 + 3t + 2 = ( t + 1 )( t^2 + t + 2 ) = 0$ . Solving,

$t = -1 rArr tan ( x/2 ) = - 1 = tan ( -pi/4 )$ that gives solution in

multitude as

$x/2 = kpi - pi/4 rArr x = 2kpi- pi/2 = (4k - 1 )pi/2$ ,

$k = 0, +-1, +-2, +-3, ...$ .

Also,

#t^2 +t + 2 = 0, giving t = complex ( unreal ) values, and so,

there are no other solutions.

Graphical depiction, in t-intercept t = -1, for unique solution of

$t^3 + 2t^2 + 3t + 2 = 0$ .
graph{y-x^3-2x^2-3x-2=0}