Go adding lines to a '.txt' and then print them - Android Studio

I have three Activitys, the main one:

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    String[] archivos = fileList();

    if (!existe(archivos, "notas.txt")) {
        crear();
    }
}

public void calcular(View v){
    Intent i = new Intent(this, Calcular.class);
    startActivity(i);
}
public void ver(View v){
    Intent i = new Intent(this, Ver.class);
    startActivity(i);
}

private boolean existe(String[] archivos, String archbusca) {
    for (int f = 0; f < archivos.length; f++)
        if (archbusca.equals(archivos[f]))
            return true;
    return false;
}

private void crear(){
    try {
        OutputStreamWriter archivo = new OutputStreamWriter(openFileOutput(
                "notas.txt", AppCompatActivity.MODE_PRIVATE));
    }catch(IOException e){}
}
}

One that writes the results of an operation in a '.txt':

public class Calcular extends AppCompatActivity {
private EditText et1, et2;
private RadioButton rb1, rb2, rb3, rb4;
private int r=3;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_calcular);

    et1 = findViewById(R.id.et1);
    et2 = findViewById(R.id.et2);
    rb1 = findViewById(R.id.rb1);
    rb2 = findViewById(R.id.rb2);
    rb3 = findViewById(R.id.rb3);
    rb4 = findViewById(R.id.rb4);
}

public void operar(View v){
    int a, b;
    a = Integer.parseInt(et1.getText().toString());
    b = Integer.parseInt(et2.getText().toString());

    if(rb1.isChecked()){
        r = a + b;
    }
    else if(rb2.isChecked()){
        r = a - b;
    }
    else if(rb3.isChecked()){
        r = a * b;
    }
    else if(rb4.isChecked()){
        r = a / b;
    }

    try {
        FileWriter archivo = new FileWriter(getFileStreamPath("notas.txt"), true);
        archivo.write( String.valueOf(r) );
        archivo.flush();
        archivo.close();
    } catch (IOException e) {
    }
}
public void volver(View v){
    finish();
}
}

And the other should read and print the results that are accumulating:

public class Ver extends AppCompatActivity {
private TextView tv1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_ver);

    tv1 = findViewById(R.id.tv1);

    try{
        InputStreamReader archivo = new InputStreamReader(
                openFileInput("notas.txt"));
        BufferedReader br = new BufferedReader(archivo);
        String linea = br.readLine().toString();
        String todo = "";
        while(linea != null){
            todo = todo + linea;
            linea = br.readLine().toString();
        }
        br.close();
        archivo.close();
        tv1.setText(todo);
    }catch(IOException e){}

}
public void volver(View v) {
    finish();
}
}

But only write the last result. How can I solve that? Thanks.

Could it be that the write () method is overwriting the values that were stored?