Mathematical definition of critical point？(thermodynamics)

[kof9595995]

Homework Statement

If a gas is described by Van der Waals eqn (p+a/v^2)(v-b)=kT, find the critical pint p_c, T_c,and v_c

Homework Equations

The Attempt at a Solution

Actually this is just one intermediate step I got stuck for my stat mech homework, I don't even know the mathematical definition for critical point(I didn't learn quite well in my thermodynamics course), the only relevant equation I found is in wiki http://en.wikipedia.org/wiki/Critical_point_(thermodynamics), but I don't think it is adequate for my problem.

 

[kof9595995]

Em.. sorry, I made a silly mistake in my mind, equations given in wiki should be adequate for my problem.