Example $3$: $($P$2\mathrm{.}57$ White$)$

The tank is $2$ m wide into the screen. Neglecting the atmospheric pressure, find the resultant hydrostatic force on panel BC $($a$)$ a single formula, $($b$)$ by computing horizontal and vertical forces separately $($this is the method also used for curved surfaces, that is why it is asked here$).$