If Δ(x) =

|f1(x) g1(x)|

|f2(x) g2(x)|

its differentiatin Δ'(x) =

|f1'(x) g1'(x)|

|f2(x) g2(x)|

Plus

|f1(x) g1(x)|

|f2'(x) g2'(x)|

This means to differentiate a determinant, we differentiate one row at a time, keeping others unchanged and then add them

If there are two rows R1 and R2

Δ'(x) =

|R1'|

|R2|

Plus

|R1|

|R2'|

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