Hello,
In Example 27 of Vector , we have to find angle between two lines... and formula of angle has cos, that's why we used cos.
Please check it here https://www.teachoo.com/3459/748/Example-27---If-i-j-k--2i-5j--3i-2j-3k-and-i-6j-k-are-position/category/Examples/
Written on Sept. 3, 2017, 9:17 p.m.