A particle is describing uniform circular motion in the anti-clockwise sense such that its time period of revolution is $T$. At $t=0$ the particle is at $A$. If $\theta_1$ is the angle between acceleration at $t=T/4$ and average velocity in the interval $0$ to $T/4$, and $\theta_2$ is the angle between acceleration at $t=T/4$ and the change in velocity in the interval $0$ to $T/4$, then