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THS4561: how to get the fillter's cut-off frequency?

Part Number: THS4561

in THS4561 datasheet,the 1th page,the picture below is the demo shematic of butterworth this circuit,how to caculate it's cut-off frequency?i found use F=1/(2*PI*sqrt(R1*C1*R2*C2)) can't get correct value.thanks a lot!