Math Help - Dot Product Question

1. Dot Product Question

Maybe I'm overlooking the simplicity of the question, but I keep coming up with answers separate to the given answer.

The angle between $\vec a$ and $\vec b$ is $79 deg$.

Find $p$ if $\vec a = 6\vec i + 3 \vec j - 2 \vec k$ and $\vec b = -2 \vec i + p \vec j - 4\vec k$

2. Originally Posted by gordonparsons
Maybe I'm overlooking the simplicity of the question, but I keep coming up with answers separate to the given answer.

The angle between $\vec a$ and $\vec b$ is $79 deg$.

Find $p$ if $\vec a = 6\vec i + 3 \vec j - 2 \vec k$ and $\vec b = -2 \vec i + p \vec j - 4\vec k$

We know (hopefully) that $\cos 79 = \frac{\vec a\cdot \vec b}{||\vec a\|\|\vec b\|}$ ....well, do the math.