Here in the Figure R5=50K. This resistor value is not in the equations to obtain output voltage, and in many tutorials non-inverting input is directly grounded without a resistor. What might be the reason to use it here?
Answer
R5 equals R4 || R6 (resistors in parallel) calculated as $$R_5 = \frac{R_4 \cdot R_6}{R_4 + R_6}$$ and its purpose is to balance the two inputs of the opamp, it cancels the differences caused by the opamp input bias current
http://www.ecircuitcenter.com/Circuits/op_ibias/op_ibias.htm
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