Que Tal Marco...

: If an audio signal (cosine) with 4 KHz of frequency and 10 V of
: amplitude modulates a carrier of 100 MHz, with a percentual modulation
: of 85%, does the carrier have 11.76 V of amplitude?

If you use a carrier of 100MHz, we would assume you to be in the common US
FM Broadcast band. Many people refer to the audio signal as a sine wave.
(Unless we want to talk cofunctions... probably not.)

: Supposing that the transmitter power is of 1 kW how can I know the
: power of the carrier and sidebands? How can I know this values in
: Watt, dBW and dBm?

At 100% modulation or at 85%..?

You can know the values by assuming a reference or knowing what the
reference is. dB might assume a starting point, reference power level
supplied/measured by you.

dBm often refers to a power level referenced to a milliwatt value across a
termination resistance. Other examples to research might be dBc, dBrnc and
similar labels.

: If I just hadn't fluke the exame...
: Thank everybody in advance, Marco.

Assuming you are working with an AM signal, one of... if not the most
accurate way to measure the complex output signal would be with a scope
properly connected.

The ARRL Handbooks provide a measure of reference information... some of
the vintage handbooks are better at describing AM which was most popular
in the 50's through the 70's.

cheers
skipp

(el gusto es mio...)