From the previous sections we've learnt that:

So: Z_L+C = X_Lj - X_Cj = (X_L - X_C)j. The impedance is purely imaginary.

|Z_L+C| = |X_L - X_C|.

This means that the total impedance is less than the impedance of each single component. If X_L and X_C are equal, the impedance will be zero! This can also be explained in an other way. The current flow through both components is the same, while the voltage across one component has a 90 degrees phase shift and the voltage across the other component a -90 degrees phase shift. The phase shift between the voltages across C and L will be 180 degrees. Because the absolute impedances are the same, the voltage amplitudes will also be the same. So both voltages will 'rule each other out'. The total voltage across L and C will be 0V. This means that the impedance is 0Ω.