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The condition for -3 dB point is R = Xc, Xc being capacitive reactance magnitude. But why is attenuation of RC voltage divider 0.7 rather than 0.5 for equal magnitude of both impedances? That's due to phase shift. Resistor and capacitor voltage are 90 degree phase shifted, relation of voltages can be visualized by phasors, both phasors form a right angled triangle, their magnitudes add as squares. |Vr + Vc| = 1.41 * |Vr| = 1.41 * |Vc|.