What caution concerning H$1($f$)$H$1($f$)$ must be considered?

For these formulas to be valid, H$1($f$)$H$1($f$)$ must be the transfer function magnitude of the first filter with the second detached.For these formulas to be valid, H$1($f$)$H$1($f$)$ must be the transfer function of the first filter with the second detached.For these formulas to be valid, H$1($f$)$H$1($f$)$ must be the transfer function of the first filter with the second attached.For these formulas to be valid, H$1($f$)$H$1($f$)$ must be the transfer function magnitude of the first filter with the second attached.

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