In the web page of camera calibration there are some mistakes, which puzzle fresh guys.
http://opencv.itseez.com/doc/tutorials/calib3d/camera_calibration/camera_calibration.html

In Section "Theory"

For the radial distortion: miss ')' in each formula.
For the tangent distortion, should modify as follows:
(1) miss 'x' in the first correct term
<pre>
x_{correct} = x + [2p_{1}y + p_{2}(r^2+2x^2)] ====> x_{correct} = x + [2p_{1}xy + p_{2}(r^2+2x^2)]
</pre>

(2) no '2' in the first correct term and miss 'y' in the second term
<pre>
y_{correct} = y + [2p_{1}(r^2+2y^2) + 2p_{2}x] ====> y_{correct} = y + [p_{1}(r^2+2y^2) + 2p_{2}xy]
</pre>