Hey, thank you so much, I think u are right, but still, ahaha, maybe it's me but new problems here!
Max: 2n\cos(\theta_1) = (2m+1)
it becomes cos(\theta_1)=\frac{2m+1}{3}
The first solution i found is when m=1, indeed
cos(\theta_1)=1 so that
\theta_1=0°,
now, to find the...