title: Get Value from two form control and set it to one form control
body: <p>How could I set a value of form control after calculating from two form Control?
  These controls are part of the same form group.  And the form group is dynamically added to the form array.</p>
  <p><a href="https://i.stack.imgur.com/kL0Ye.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kL0Ye.png" alt="enter image description here" /></a></p>
  <pre><code>           this.purchaseForm = this.fb.group({
            purchases: this.fb.array([this.getPurchaseFormGroup()])
          });
        
        getPurchaseFormGroup(): FormGroup {
          const form = new FormGroup({
            itemName: new FormControl(''),
            itemQty: new FormControl(''),
            itemPrice: new FormControl(''),
            itemAmt: new FormControl('')
          });
          return form;
      
        }
  
  public addItem() {
      const form = this.purchaseForm.get('purchases') as FormArray
      form.push(this.getPurchaseFormGroup());
    }
  </code></pre>