ts-sdk: fix client borrow interest rate calculation (#479)

* bigz/fix-calculateInterestRate * update CHANGELOG.md --------- Co-authored-by: 0xbigz <“[email protected]”>
drift-labs · May 25, 2023 · 5cf9b27 · 5cf9b27
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - program: add user perp lp jit liquidity toward a target base (([#448](https://github.com/drift-labs/protocol-v2/pull/448)))
 - ts-sdk: drift client will query rpc to find all markets/oracles if they're not explicitly specified (([#469](https://github.com/drift-labs/protocol-v2/pull/469)))
+- ts-sdk: fix client borrow interest rate calculation (([#479](https://github.com/drift-labs/protocol-v2/pull/479)))
 
 ### Fixes
 

diff --git a/sdk/src/math/spotBalance.ts b/sdk/src/math/spotBalance.ts
@@ -282,9 +282,7 @@ export function calculateInterestRate(bank: SpotMarketAccount): BN {
 	} else {
 		const borrowRateSlope = new BN(bank.optimalBorrowRate)
 			.mul(SPOT_MARKET_UTILIZATION_PRECISION)
-			.div(
-				SPOT_MARKET_UTILIZATION_PRECISION.sub(new BN(bank.optimalUtilization))
-			);
+			.div(new BN(bank.optimalUtilization));
 
 		interestRate = utilization
 			.mul(borrowRateSlope)