Fourier series in continuous time domain while representing $a_k$ in rectangular form $$ a_k = B_k + jC_k$$ But when using the value of $a_k$ in the main equation: $$ x(t) = a_0 + 2\sum^{+\infty}_{k\ =\ 1} [B_k\cos k\omega_0t - jC_k\sin k\omega_0t]$$
I want to ask from where $-$(minus) sign comes as there was no minus sign main Equation $$ x(t) = a_0 + 2\sum^{+\infty}_{k\ =\ 1} 2\mathrm{Re}\left[ e^j(k \omega_0t +\theta_k)\right]$$