Finding the acute angle between two lines

[Monkeyseat]

Hi,

Question

Working

I only need help with part (c). This is the book's solution for part (c):

I don't understand where they got "cos theta = 1/2" from or their answer of 60 degrees. If you look at the bit I higlighted in green, I solved that for theta and got an answer of 30 degrees.

Could someone please clarify this answer?

Thanks.

 

[DrMike]

Seems like an error in the textbook.

 

[Monkeyseat]

Thanks for clarifying. It had me confused!