Use (i) the Ampere’s law for H and (ii) continuity of lines of B,

C is the amperian loop which is given as

$\int_{Q}^{P} \vec{H} \cdot \overrightarrow{d l}=\int_{Q}^{P} \frac{\vec{B}}{\mu_{0}} \cdot \overrightarrow{d l}$

Solving the above equation we get the angle between

$\vec{H}$ and $\overrightarrow{d l}$ more than $90^{\circ}$ so that $\cos \theta$ is negative.