Look at these adjectives:
Tom est beau, mais Henri et Bob sont très beaux.Tom is handsome, but Henri and Bob are very handsome.
Yvette est belle, mais Leila et Belinda sont très belles.Yvette is beautiful, but Leila and Belinda are very beautiful.
C'est un bel animal parmi de beaux animaux.It's a beautiful animal among beautiful animals.
Note that to form the masculine plural, adjectives ending in -eau add -x, and vieux doesn't change.
As for the feminine plural, it is regular and simply takes an -s.
The masculine forms bel, nouvel and vieil (when in front of a vowel) become the regular masculine plural beaux, nouveaux and vieux.
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