Collapsing button's class changes when item is collapsed or not - html

Question

sorry for this code but I'm just beginning developing and don't know at all where is the trigger for what I want.
so what I am looking for :
I already achieved that when I click on a button, the area connected to it shows and the 2 others collapse.
What I don't know, is HOW I can change the class of the button (from fa-angle-right to fa-angle-down) when my area is shown and get back to the default when collapsed...
here is my HTML :
<section id="mes-creations" class="mes-creations text-center">
<div class="mes-creations">
<div class="container">
<div class="row">
<div class="col-lg-12">
<h2 id="titre-section">section title</h2>
<p>section description</p>
</div>
</div>
<!--bouttons-->
<div class="row">
<p>
<div class="col-sm-4 col-md-4 col-lg-4">
<a class="btn btn-primary stylebtn collapsed" id="bouton-1" data-toggle="collapse" data-parent="#accordeon" href="#collapseGallery1" aria-expanded="false" aria-controls="collapseGallery1">
<span class="fa fa-angle-right animated"> bouttontitle1</span>
</a>
</div>
</p>
<p>
<div class="col-sm-4 col-md-4 col-lg-4">
<a class="btn btn-primary stylebtn collapsed" id="bouton-2" data-toggle="collapse" data-parent="#accordeon" href="#collapseGallery2" aria-expanded="false" aria-controls="collapseGallery2">
<span class="fa fa-angle-right animated"> boutontitle2</span>
</a>
</div>
</p>
<p>
<div class="col-sm-4 col-md-4 col-lg-4">
<a class="btn btn-primary stylebtn collapsed" id="bouton-3" data-toggle="collapse" data-parent="#accordeon" href="#collapseGallery3" aria-expanded="false" aria-controls="collapseGallery3">
<span class="fa fa-angle-right animated"> boutontitle3</span>
</a>
</div>
</p>
</div>
<!--end button-->
<!--gallery-->
<p>
<div class="row" id="accordeon">
<div class="gallery-custom row collapse" id="collapseGallery1">
<div class="col-xs-6 col-sm-4 col-md-3"><img class="img-gallery-style" src="img/Banniere/11-fleurs.png" alt="Banniere1"></div>
<div class="col-xs-6 col-sm-4 col-md-3"><img class="img-gallery-style" src="img/Banniere/Annees-80.png" alt="Banniere2"></div>
<div class="col-xs-6 col-sm-4 col-md-3"><img class="img-gallery-style" src="img/Banniere/Banniere-robo.gif" alt="Banniere3"></div>
<div class="col-xs-6 col-sm-4 col-md-3"><img class="img-gallery-style" src="img/Banniere/banniere-site-2012.png" alt="Banniere4"></div>
<div class="col-xs-6 col-sm-4 col-md-3"><img class="img-gallery-style" src="img/Banniere/Collection-western.png" alt="Banniere5"></div>
<div class="col-xs-6 col-sm-4 col-md-3"><img class="img-gallery-style" src="img/Banniere/Beyblade-Film-V1G.png" alt="Banniere6"></div>
</div>
<div class="gallery-custom row collapse" id="collapseGallery2">
<div class="col-xs-4 col-sm-3 col-md-3"><img class="img-gallery-style" src="img/CP/Camelot-CP.png" alt="CP1"></div>
<div class="col-xs-4 col-sm-3 col-md-3"><img class="img-gallery-style" src="img/CP/CP-Groupe-delite-Final.png" alt="CP2"></div>
<div class="col-xs-4 col-sm-3 col-md-3"><img class="img-gallery-style" src="img/CP/CP-guerre-des-empires.png" alt="CP3"></div>
<div class="col-xs-4 col-sm-3 col-md-3"><img class="img-gallery-style" src="img/CP/CP-GUERRIERE_ZYLO.png" alt="CP4"></div>
<div class="col-xs-4 col-sm-3 col-md-3"><img class="img-gallery-style" src="img/CP/CP-MAntera-copie.png" alt="CP5"></div>
<div class="col-xs-4 col-sm-3 col-md-3"><img class="img-gallery-style" src="img/CP/CP-OFF-WHITE-LIES_ZYLO2.png" alt="CP6"></div>
</div>
<div class="gallery-custom row collapse" id="collapseGallery3">
<div class="col-xs-6 col-sm-6 col-md-6"><img class="img-gallery-style" src="img/Print/BLACKS-GAME-ANNEES-LASER.png" alt="Print1"></div>
<div class="col-xs-6 col-sm-6 col-md-6"><img class="img-gallery-style" src="img/Print/BLACKS-GAME-MADMOVIES.png" alt="Print2"></div>
<div class="col-xs-6 col-sm-6 col-md-6"><img class="img-gallery-style" src="img/Print/print-2013-final.png" alt="Print3"></div>
<div class="col-xs-6 col-sm-6 col-md-6"><img class="img-gallery-style" src="img/Print/Print-Line-Up-Final.png" alt="Print4"></div>
</div>
</div>
</p>
</div>
</div>
</section>
and my js :
$('#accordeon').on('show.bs.collapse', function () {
$('#accordeon .in').collapse('hide');
});
Demo

Answer 1

Here's a somewhat hackish way to do it. It would be simpler if you were using markup structure closer to what Bootstrap suggests.
$('.stylebtn').click(function() {
$(this).find('.fa').toggleClass('fa-angle-down fa-angle-right')
.closest('div').siblings().find('.fa')
.removeClass('fa-angle-down').addClass('fa-angle-right');
});
Demo