Change of base: log_a b = 1 / log_b a

CAT has tested this idea 5 times across 5 papers, from 2018 to 2021. Solve one, and the others are the same method with different numbers or wording, which is why repeated concepts are the most efficient thing to practise.

Question	What it asks	Answer
CAT 2021 Slot 3 · QA Q6	sum of reciprocals of log_15 a and log_32 a	A
CAT 2018 Slot 2 · QA Q34	alternating sum of 1/log_k 100	B
CAT 2020 Slot 2 · QA Q5	log_a(a/b) + log_b(b/a) cannot equal	B
CAT 2020 Slot 3 · QA Q3	express log_3 a using given logs	A
CAT 2018 Slot 1 · QA Q12	log_12 81 = p; simplify expression in p	B

Part of Logarithms & Exponents (55 questions since 2017).