user10465: I am a new [and a late] comer to chess. I started practicing on chessacademy.com where I got this exercise:
r1bqk1nr/ppp2pp1/4p2p/8/1bPnN3/5N2/PP1B1PPP/R2QKB1R w KQkq - 1 13

According to the site, the best move is ♗b4 and then it asks the following:

[Why not accomplish the same thing by Qa4+...Qxb4?]

I can't see why not? Any hints?

user3456: This kind of question can be answered by using an online engine connected with a chessboard editor. Since you need to get the FEN to enter this position here you might as well use that editor and play it against computer: http://www.apronus.com/chess/playcomputer/?fen=0r1bqk1nrXppp2pp1X4p2pX8X1bPnN3X5N2XPP1B1PPPXR2QKB1R_w_KQkq_-_0_1
The game will go 1.Qa4+ Bd7 2.Qxb4 Nc2+
Hope this helps next time.

user2042: You are missing the knight fork on c2. After 1.Qa4+ Bd7 2.Qxb4 Nc2+ 3.Kd1 Nxb4 you would lose the queen...

10465: <p>I am a new [and a late] comer to chess. I started practicing on <a href="https://www.chesscademy.com" rel="nofollow">chessacademy.com</a> where I got <a href="https://www.chesscademy.com/tactics/15083" rel="nofollow">this exercise</a>:</p>

<pre><code>r1bqk1nr/ppp2pp1/4p2p/8/1bPnN3/5N2/PP1B1PPP/R2QKB1R w KQkq - 1 13
</code></pre>

<p>According to the site, the best move is ♗b4 and then it asks the following:</p>

<blockquote>
  <p>[Why not accomplish the same thing by Qa4+...Qxb4?]</p>
</blockquote>

<p>I can't see why not? Any hints?</p>
 3456: <p>This kind of question can be answered by using an online engine connected with a chessboard editor. Since you need to get the FEN to enter this position here you might as well use that editor and play it against computer: <a href="http://www.apronus.com/chess/playcomputer/?fen=0r1bqk1nrXppp2pp1X4p2pX8X1bPnN3X5N2XPP1B1PPPXR2QKB1R_w_KQkq_-_0_1" rel="nofollow">http://www.apronus.com/chess/playcomputer/?fen=0r1bqk1nrXppp2pp1X4p2pX8X1bPnN3X5N2XPP1B1PPPXR2QKB1R_w_KQkq_-_0_1</a></p>

<p>The game will go 1.Qa4+ Bd7 2.Qxb4 Nc2+</p>

<p>Hope this helps next time.</p>
 2042: <p>You are missing the knight fork on <code>c2</code>. After <code>1.Qa4+ Bd7 2.Qxb4 Nc2+ 3.Kd1 Nxb4</code> you would lose the queen...</p>
