take ln to both sides,
http://upload.wikimedia.org/math/2/6/0/26019282cbb228bcc52c1f042be4373f.png
differential both sides,
http://upload.wikimedia.org/math/3/0/c/30cc9dcaf834fe46a703cac87905ca4c.png
http://upload.wikimedia.org/math/7/4/0/7407d6d75fb55c0cbf308cdbf34b0fd0.png
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