FLT CLOSE TO ARSA BOUNDARY CAUSES TCASII RA FOR ACR WITHIN THE ARSA.

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THIS WAS A XCOUNTRY TRAINING FLT FROM RIALTO; CA; TO SNA; CA VIA PDZ VOR (RETURN FLT). AFTER DEPARTING RWY 240 AT RIALTO WITH A L XWIND DEP; MY STUDENT WAS TO INTERCEPT THE 010 DEG RADIAL FROM PDZ AND TRACK INBOUND TO PDZ. HE WAS SLOW HAD HAVING DIFFICULTY GETTING ORIENTED TO THE TASK. BY THE TIME HE RESOLVED HIS PROBS HE HAD PASSED THE 010 DEG RADIAL AND WAS PARALLELING IT. I SUGGESTED THEN PROCEEDING DIRECT TO PDZ BY CTRING THE CDI AND FLYING THE COURSE ON THE OBS (180 DEG TO). THIS HE DID WITH THE CDI SLIGHTLY TO THE R. I FELT COMFORTABLE THAT WE WERE WELL OUTSIDE THE INNER CORE OF THE ONT ARSA AND ALSO BELOW THE OUTER CORE AT 2400 FT. AS WE CONTINUED SBOUND WE CONTACTED RIVERSIDE TWR ADVISING THEM WE WERE TRANSITIONING THE W EDGE OF THEIR ATA N TO S. THEY APPROVED THE TRANSITION AND ASKED WHERE WE HAD DEPARTED FROM. WE TOLD THEM RIALTO AND THEY INFORMED US THAT ONT HAD ADVISED THEM OF A POSSIBLE AIRSPACE CONFLICT WITH AN ACFT ON OUR RTE OF FLT -- AN AIRLINER HAD A TCASII ALERT ON FINAL INTO ONT ARPT. WHILE XING THE FINAL APCH PATH TO ONT AT 2400 FT; 6 NM E; I HAD POINTED OUT TO MY STUDENT A LARGE ACFT DSNDING TO ONT ABOUT 600 FT ABOVE US. I DO NOT BELIEVE WE VIOLATED ANY AIRSPACE BUT IT IS MY RESOLVE TO INCREASE MY PERSONAL MARGINS AROUND ARSA'S TO 3-4 MI AND 500 FT BELOW.