How to make a custom JS file load within my WP child theme

Hi

I am trying to load a custom JS file in the header.php of my custom child theme. But that JS is not being loaded....

<?php echo '<script type="text/javascript">'; ?>
<?php echo get_stylesheet_directory() + "/lib/js/jquery.validate.js" ?>
<?php echo '</script>'; ?>

What is wrong with above code?

Thanks
Arvind.