import { FormControl, FormGroup, FormBuilder } from '@angular/forms';
//..

export class TriangleComponent implements OnInit {

  triangleForm ! : FormGroup;

  constructor(private formBuilder: FormBuilder) { }

  ngOnInit(): void {
    this.triangleForm = this.formBuilder.group( {
      base: [''],
      height: [''],
      area: ['']
    });    
  }