I don't understand the algebra in this answer

[pkc111]

Homework Statement

At what average rate does the magnetic field need to change to induce emf of 1.0 V.
See attachment question 1.

Relevant Equations

See attachment solution.

Why does Δ(BA) = AΔ(B) when A is constant?
Is there a proof for this algebra?
Thanks

 

[haruspex]

Δ(AB) = (A+ΔA)(B+ΔB)-AB = AΔB+BΔA+ΔAΔB.
If A constant then ΔA=0, leaving only AΔB.

 

[pkc111]

Thank you haruspex.