Question: The math formula in VariableInterestRate.sol

[anhthii]

Hi everyone, I'm learning about how fraxlend works and I'm trying to understand the math formula in this method

fraxlend/src/contracts/VariableInterestRate.sol

Line 63 in 0f9bc5d

function getNewRate(bytes calldata _data, bytes calldata _initData) external pure returns (uint64 _newRatePerSec) {

I checked the documentation but it only shows the formula of Linear Rate model but no formula for Time-Weighted Variable Interest Rate.

I looks up on the internet but couldn't find any information about this formula. Is this some formula that the team comes up with or it comes from traditional finance?

Thank you very much if any one can help me understand it