calculate cos^2 - sin^2 in model

[KyleQianliMa]

Short description of the changes:

Long description of the changes:

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

References

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (c219107) to head (9b5d2cc).
Report is 2 commits behind head on next.

Additional details and impacted files

@@           Coverage Diff           @@
##             next      #42   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files          10       10           
  Lines         806      809    +3     
=======================================
+ Hits          794      797    +3     
  Misses         12       12           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.