How to calculate gain?

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The gain is determined by a bridge of R1-R4 that is intentionally unbalanced by R5.

How come the gain becomes 30X? How to calculate the gain for this "intentionally unbalanced R1-R4 bridge"?
 

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The gain can be determined by solving the circuit equations. Neglect the effects of L1 and R13, and assume an ideal op amp.
The equations are:

(Vin-Vplus)/R1 = (Vplus-Vout)/R2

Vplus=Vout*Rp/(Rp+R3)

Where Vplus is the voltage at the op amp input terminals. (For an ideal op amp the voltages are the same at both input terminals.) Rp is the parallel combination of R4 and R5.

Solving the above equations gives:

Vout/Vin=R2*(Rp+R3)/(Rp*R2-R1*R3)

The gain for this circuit is 32.
 
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