使用 fastboot oem unlock来对nexus 9解锁 .

http://www.droid-life.com/2013/11/04/how-to-unlock-the-nexus-5-bootloader/
http://www.inexus.co/thread-18503-1-1.html http://www.ibtimes.co.uk/how-root-google-nexus-9-unlock-bootloader-1473817
开启程序员选项，然后在开启 oem 解锁功能

-使用 fastboot oem unlock来对nexus 9解锁 ...

1. Enable “Developer options” by tapping 5 or 6 times on the Build number in Settings>About phone.

2. Hit back once and choose Developer options; check the box for USB debugging.

3. Plug your phone into your computer.

4. You’ll need to give it USB debugging access (don’t forget to check the box).

5. Once you have given your computer debugging access, it’s time for commands. Type in a command prompt:

adb reboot bootloader

(Note: If using a Mac, it’s ./adb reboot bootloader or ./fastboot -insert comand-)

6. Wait for your bootloader screen to appear. Once it does (big Android with START), type:

fastboot oem unlock

7. A bootloader unlock confirmation page will appear. Tap Volume Up to highlight “Yes,” and Power to select it.

8. Your phone will then be erased and the bootloader unlocked. Shouldn’t take more than a minute or two.