The document derives an integral formula for sec3x in 3 steps. It first shows that the integral of secx dx equals ln|secx + tanx| + C. It then rewrites the integral of sec3x dx in terms of secx and tanx. Finally, it integrates by parts and combines the results with the first step to obtain the formula: ∫ sec3x dx = 1/2 ln|secx tanx| + tanx secx + C.