Also, neither the Itute nor TL solns provided a satisfactory explanation for why the negative

solution was invalid in Q14c(ii).

Itute gave no reasoning for selection of the +ive inner surd, while TL reasoned that cos^2() must be positive (but clearly the [

] solution is also positive).

My reasoning is that where we subst,

,

in order to get the polynomial, we could equally well have substituted

,

to get the same equation.

So the polynomial is equally valid for solving for,

, as well as some other non acute angles (which clearly can be ignored).

So the two acute solutions,

correspond to solutions for cos(pi/10) and cos(3pi/10), and since cos() is a decreasing function on (0,pi), then the larger value must correspond to cos(pi/10).

Did anyone else use this reasoning?