Additional costs

[Jscroats31]

if I have a card such as Skaab goliath that I want to cast that has an ADDITIONAL cost of "exile two creature cards from your graveyard" but I don't have any creatures in my graveyard can I still cast it even though I don't have the resources to pay for its additional cost?

[MuggyWuggy]

If the additional cost is required  you cannot cast it without paying the required cost
{Skaab Goliath}

[Jscroats31]

Yea but how do you specify if an additional cost is required?

[Kaworu, the Fifth Child]

Unless it says "you may" assume it's nessessary.