Maths Question

https://www.quora.com/profile/Bravewarrior/p-143521002
Here is the question and solution. I need help with part b. I don't understand why at the end they have negative root 3 over 4. I thought that as cos squared theta = 3/4 so by square rooting both sides therefore cos theta =plus minus root 3 over 2. Some help would be great!

Agree with you. It's a well known trig value.

I guess the solution is wrong...😭

Yup. It's a 1:sqrt(3):2 right triangle and you can read the trig values off that.