Ans. The change of base rule should be used in JEE problems when we need to evaluate logarithms with bases that are not easily calculable or manipulate logarithmic expressions algebraically. The change of base rule allows us to convert logarithms with one base to logarithms with a different base by dividing the logarithm of the number with the old base by the logarithm of the new base. This can be particularly useful when we want to evaluate logarithms using a calculator that only supports certain bases, such as base 10 or base e (natural logarithm). Additionally, the change of base rule can help in simplifying expressions or performing algebraic manipulations involving logarithms with different bases.