install OpenCV for python3 in Jetson Nano

sorry, in my jetson nano:

alex@alex-jetson-nano:~$ python3
Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
ImportError: numpy.core.multiarray failed to import
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: numpy.core.multiarray failed to import

I also create a virtual python environment tensorflow-gpu, when import cv2:

alex@alex-jetson-nano:~$ workon tensorflow-gpu
(tensorflow-gpu) alex@alex-jetson-nano:~$ python3
Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'cv2'