Numerical Integration
Determine a formula \[ \int _{-\pi }^{\pi } f(x)\cos (x)\,dx \approx A_0 f\left (-\frac {3}{4}\pi \right ) +A_1 f\left (-\frac {1}{4}\pi \right ) +A_2 f\left (\frac {1}{4}\pi \right ) +A_3 f\left (\frac {3}{4}\pi \right ) \] that is exact when \(f\) is a polynomial of degree 3.