Question

Find II 2e-3f II^2 assuming that e & f are unit vectors such that II e +f II=sqrt(3/2).

Answer 1

We are given that ||e|| = 1, ||f|| = 1. 

Since ||e + f|| = sqrt(3/2), we have 
3/2 = (e + f) dot (e + f) 
= (e dot e) + 2(e dot f) + (f dot f) 
= ||e||^2 + 2(e dot f) + ||f||^2 
= 1^2 + 2(e dot f) + 1^2 
= 2 + 2(e dot f). 

So e dot f = -1/4. 

Therefore, 
||2e - 3f||^2 = (2e - 3f) dot (2e - 3f) 
= 4(e dot e) - 12(e dot f) + 9(f dot f) 
= 4||e||^2 - 12(e dot f) + 9||f||^2 
= 4(1)^2 - 12(-1/4) + 9(1)^2 
= 4 + 3 + 9 
= 16.