一个小型加法计算器，在输入框里输入数据，键入button后进入下一个activity显示了计算结果，这时候按下返回键，输入过的数据还会输入框在里面。请问，如何在按返回的同时，清空原有文本框EditView中输入过的内容，让我在下一次输入的时候，输入框里是空白？

此段为第一个页面

package dhu.qq.com;

import android.R.integer;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class ZuoyeActivity extends Activity {
/** Called when the activity is first created. */
private EditText et1;
private EditText et2;
private Button bt1;
private Float f1, f2;

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

et1 = (EditText) findViewById(R.id.et1);
et2 = (EditText) findViewById(R.id.et2);
bt1 = (Button) findViewById(R.id.bt1);

bt1.setOnClickListener(new OnClickListener() {

@Override
public void onClick(View v) {
// TODO Auto-generated method stub
f1 = Float.parseFloat(et1.getText().toString());
f2 = Float.parseFloat(et2.getText().toString());

Bundle bd1 = new Bundle();
bd1.putFloat("iput1",f1);
bd1.putFloat("iput2",f2);

Intent itt1 = new Intent();
itt1.putExtra("transfer_bundle", bd1);

itt1.setClass(ZuoyeActivity.this, SecondyActivity.class);
startActivity(itt1);

}
});
}
}

此段为第二个页面

/**
*
*/
package dhu.qq.com;

import java.security.PublicKey;

import android.R.string;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.TextView;

/**
* @author Administrator
*
*/
public class SecondyActivity extends Activity {

/*
* (non-Javadoc)
*
* @see android.app.Activity#onCreate(android.os.Bundle)
*/
private TextView tv21;
private Float f1, f2, f3;
private String str;

@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.main2);
tv21 = (TextView) findViewById(R.id.tv21);

Intent itt2 = new Intent();
Bundle bd12 = new Bundle();

itt2 = this.getIntent();
bd12 = itt2.getBundleExtra("transfer_bundle");
f1 = bd12.getFloat("iput1");
f2 = bd12.getFloat("iput2");
f3 = f1 + f2;

str = String.valueOf(f3);
tv21.setText(str);

}
}