I consider "magic resist' = "Fire+Ice+Shock resist",however,when I equipted Lydia with a boot of 60% ice resist(plus her nord perk,110% of ice resist),I hit her with ice magic spell,the screen showed "Lydia resist that kind of spell",but when I equipted her with a shied with 20% magic resist and a boot with 45% ice resist( so 95% ice resist + 20% magic resist,I consider it =115 ice resist + 20 fire resist + 20 shock resist),then ,I hit her with ice magic spell,she still took hurt,after some hits ,she even left me alone(as a reponse of me maltreated her),but in the first case,this would not happen. So, why?How the "magic resist" works?

You can *ignore* the damage up to %85 of it since it is cap. I remember reading somewhere you can add elemental resistance up to that %85 cap.
MR is broken if you have patch 1.2 installed.

It's rumoured resistance cannot go past 85% and if true, that is why your Lydia took damage.

Resist magic and resist elements are used separatelly. So while 50 resist frost + 50 resist frost make you inmune to frost, 50 resist frost + 50 resist magic make you take 25% of frost damage.

Also, as others say, magic resistance is caped, while elemental resistance not.

I'm very sure that elmental resistance has no cap(at least in 1.1 version),cause Lydia can ignore ice magic spell(screen showed),what amgepo saied is quite resonable,can't bethesda made it clearer?

Do you mean elemental resists are additive

eg. 20% frost resist + 10% frost resist = 30% frost resist

While magic resists are multiplicative

eg. 20% frost resist + 10% magic resist = 20% + 0.1 X (100% - 20%) = 28% frost resist?

IIRC, magic resistance is applied to reduce damage before elemental resistances are applied. So if you have 50% magic resistance and 50% frost resistance, you'd take 25 damage from a 100 damage frost attack. (100 damage - 50% magic resistance = 50 damage; then 50 damage - 50% frost resistance = 25 damage).

Both are additive, but magic resistance is caped at 85% and both effects are applied separatelly, so 45 fire resistance + 33 magic resistance is not 78 resistance.

Thanks for clarifying. This explains a lot!

On my Char I Have 52% resist Frost, Fire and shock, + I get the Alteration passive magic resist. To compensate for damage taken I have 70% life regen. At 500 hp things barely dent you, except the damage can add up pretty quick if 4 mages attack you. Also there is no resist to Drain life so ancient vampires are dangerous.