15% core strength + defense bonus calculation

Neither. It only works on defense, so it’s 85%

Well the 15% offense bonus for core is already calculated when you look at the units Damage Potential. It probably does just multiply this by 0.85 when attacking a unit also in core, which would correspond to the first equation. However, that gives the defense a slight advantage, instead of canceling each other at as in the second equation. For example, say the attack before any bonuses is 1.0.

1 * 1.15 * 0.85 = 0.9775 # defense has slight advantage with regard to core bonus

where as equation 2.

1 * ( 1 + 0.15 - 0.15 ) = 1 # cancel each other out

Heh you're right, it somehow stuck in my memory that it was a defense bonus, but the wiki says it works on offense too!

I guess we'll never know about your case unless freezy is also interested and goes figuring it out, but I did a test on a Comintern L3 armored car just now.

The original attack strength against infantry is 5.9 In the city where it was built, in the core, it was reported as 6.8 - a rounded 15% bonus. When it drove onto a plain, it was reported as 10.2 - exactly 50% extra on the 6.8. So my guess is, these bonuses are multiplicative and not additive. Not enough evidence to say if this applies to all cases where several bonuses are involved, but a glimpse into the developer's mind.

Also note that for the extra terrain bonuses of the PanAsian doctrine, it specifically says that they are additive to the original bonus - a pointer in exactly he opposite direction.