在选择时，根据值FormArray 7更改显示div内容

<div *ngFor="let control of formArray.controls; let i=index">
  <!-- 根据索引值i判断是否显示div内容 -->
  <div *ngIf="i === 7">
    <!-- div内容 -->
  </div>
</div>

import { Component } from '@angular/core';
import { FormArray, FormControl } from '@angular/forms';

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html',
  styleUrls: ['./example.component.css']
})
export class ExampleComponent {
  formArray: FormArray;

  constructor() {
    this.formArray = new FormArray([
      new FormControl('value 1'),
      new FormControl('value 2'),
      // ...
      new FormControl('value 7'),
      // ...
      new FormControl('value n')
    ]);
  }
}

import { Component } from '@angular/core';
import { FormArray, FormControl } from '@angular/forms';

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html',
  styleUrls: ['./example.component.css']
})
export class ExampleComponent {
  formArray: FormArray;

  constructor() {
    this.formArray = new FormArray([
      new FormControl('value 1'),
      new FormControl('value 2'),
      // ...
      new FormControl('value 7'),
      // ...
      new FormControl('value n')
    ]);

    // 监听第7个控件的值变化
    this.formArray.controls[6].valueChanges.subscribe(value => {
      if (value === 'desired value') {
        // 显示div内容
      } else {
        // 隐藏div内容
      }
    });
  }
}