in previous versions of angular like Angular  5 we use to write below code in our angular.json 

"styles": [
    "styles.scss"
  ],
"scripts": [
    "../node_modules/jquery/dist/jquery.slim.min.js",
    "../node_modules/popper.js/dist/umd/popper.min.js",
    "../node_modules/bootstrap/dist/js/bootstrap.min.js"
  ],

but in Angular 6 you might get some errors like

Error: ENOENT: no such file or directory, open '/Users/pacozevallos/myApp/node_modules/jquery/dist/jquery.slim.min.js'

so to solve this here is new updated code for your angular.json 

"styles": [
   "src/styles.scss",
   "node_modules/bootstrap/dist/css/bootstrap.min.css"
],
"scripts": [
   "node_modules/jquery/dist/jquery.slim.min.js",
   "node_modules/popper.js/dist/umd/popper.min.js",
   "node_modules/bootstrap/dist/js/bootstrap.min.js"
]

it worked for me hope this will help you too.