The Wronskian & Variation of Parameters · Easy problem

To solve $y''+y=\sec t$ by variation of parameters, write $y_p=u_1\cos t+u_2\sin t$, where $u_1'=-\dfrac{y_2\,g}{W}$ and $u_2'=\dfrac{y_1\,g}{W}$ with $y_1=\cos t$, $y_2=\sin t$, $g=\sec t$. Find $u_1'$ at $t=\dfrac{\pi}{4}$.