Hello darkfenix21 
Originally Posted by
darkfenix21
First off I hope this is in the right forum
I've been working on two questions both of which have been giving me trouble. The first states:
Determine "k" given two vectors and the angle between them.
vectors a=(1,1) and b=(0,k) with and angle of 45 degrees.
I was using the dot product to solve with with
a.b= lal lbl cos45
0= (sqrt 2) + kcos45
-(sqrt 2)/cos 45 =k
k=2
if anyone could tell me if I'm right that be fantastic
I'm not sure what you mean here. The vectors $\displaystyle \binom 1 1$ and $\displaystyle \binom 0 k$ are at $\displaystyle 45^o$ for all positive values of $\displaystyle k$.
The second question is similar:
Find the value of "p" if the vectors r=(p,p,1) and s+(p,-2,-3) are perpendicular to each other
again i used the dot product and got
0=lal lbl cos90
0= (p)(p) +(p)(-2)+(-1)(3)
0= (p^2) -2p -3
3= (p^2) -2p
this is where I get stuck so if anyone could help me out it would be much appreciated thank you
Solve the quadratic equation:
$\displaystyle p^2 -2p -3=0$
You can do that, can't you?
Grandad