title: Why I can not use anguar/material elements?
body: <p>When I want to use angular materials. I got error.
  </p>
  
  <pre><code>&lt;mat-accordion&gt;
    &lt;mat-expansion-panel&gt;
      &lt;mat-expansion-panel-header&gt;
        &lt;mat-panel-title&gt;
          Personal data
        &lt;/mat-panel-title&gt;
        &lt;mat-panel-description&gt;
          Type your name and age
        &lt;/mat-panel-description&gt;
      &lt;/mat-expansion-panel-header&gt;
  
      &lt;mat-form-field&gt;
        &lt;input matInput placeholder="First name"&gt;
      &lt;/mat-form-field&gt;
  
      &lt;mat-form-field&gt;
        &lt;input matInput placeholder="Age"&gt;
      &lt;/mat-form-field&gt;
    &lt;/mat-expansion-panel&gt;
    &lt;mat-expansion-panel (opened)="panelOpenState = true"
                         (closed)="panelOpenState = false"&gt;
      &lt;mat-expansion-panel-header&gt;
        &lt;mat-panel-title&gt;
          Self aware panel
        &lt;/mat-panel-title&gt;
        &lt;mat-panel-description&gt;
          Currently I am {{panelOpenState ? 'open' : 'closed'}}
        &lt;/mat-panel-description&gt;
      &lt;/mat-expansion-panel-header&gt;
      &lt;p&gt;I'm visible because I am open&lt;/p&gt;
    &lt;/mat-expansion-panel&gt;
  &lt;/mat-accordion&gt;
  
  
  &lt;!-- Copyright 2019 Google Inc. All Rights Reserved.
      Use of this source code is governed by an MIT-style license that
      can be found in the LICENSE file at http://angular.io/license --&gt;
  </code></pre>
  
  <p>I import all related components but nothing works
  </p>
  
  <pre><code>import {
    MatAutocompleteModule,
    MatBadgeModule,
    MatBottomSheetModule,
    MatButtonModule,
    MatButtonToggleModule,
    MatCardModule,
    MatCheckboxModule,
    MatChipsModule,
    MatDatepickerModule,
    MatDialogModule,
    MatDividerModule,
    MatExpansionModule,
    MatGridListModule,
    MatIconModule,
    MatInputModule,
    MatListModule,
    MatMenuModule,
    MatNativeDateModule,
    MatPaginatorModule,
    MatProgressBarModule,
    MatProgressSpinnerModule,
    MatRadioModule,
    MatRippleModule,
    MatSelectModule,
    MatSidenavModule,
    MatSliderModule,
    MatSlideToggleModule,
    MatSnackBarModule,
    MatSortModule,
    MatStepperModule,
    MatTableModule,
    MatTabsModule,
    MatToolbarModule,
    MatTooltipModule,
    MatTreeModule,
  } from '@angular/material';
  </code></pre>
  
  <p>The error I got :</p>
  
  <blockquote>
    <p>mat-expansion-panel-header' is not a known element: 1. If
    'mat-expansion-panel-header' is an Angular component, then verify that
    it is part of this module.</p>
  </blockquote>
  
  <p>There is no document how to apply to use these elemenets. </p>