Originally Posted by

**s_ingram** Hi folks!

I joined today to find the answer to a vector question that has been driving me nuts. You know how it is! So here goes:

The two vectors **a** and **b **are of equal magnitude k (k not 0) and the angle between **a** and **b** is 60 degrees. If **c** = 3**a** - **b** and **d** = 2**a** - 10**b **

i**) **show that c and d are perpendicular

ii) find magnitudes of c and d in terms of k.

i) is OK. **c**.**d **= 0 for perpendicular vectors so evaluate (3**a** - **b**).(2**a** - 10**b**). This comes to 0 since **a**.**b** = a.b. cos 60. = k squared / 2 and a squared = b squared = k squared. (Sorry about the notation folks! - I'll figure out the superscripts later)

My problem is the second part. Can anyone help?

regards and thanks