# Angle in Biot-Savart formal in conductor loop

I am referring to this problem in Griffiths:

My question: where does cos(theta) come from? r and dl are perpendicular to each other so with the formal from Biot-Savart, dlxr (vectors) = dl and nothing more. I see the explanation in parentheses but it still doesn't make sence.

Any insight is appreciated.

• It is because you need the projection of $dB$ onto the $z$ axis. – Farcher Jun 13 at 21:07
• $d\vec{l}\times\vec{r}$ is a vector, which needs to be split into components. – orion Jun 14 at 9:09