I just don't understand the following question nor the answer. Can someone help me make sense as to what it's actually asking? Many thanks in advance.
GMAT Princeton Review 2019 Edition Drill 8 (Applied Arithmetic) Question 2
If the operation $Δ$ is defined by $a Δ b = (b² - a²) / a²$ for all numbers $a$ and$ b$, and $a ≠ 0$, then $-1 Δ (1 Δ -1) =$
$-1, 0, 1, 9, 25$
Answer $ = 25$
Explanation
The problem provides the definition of the function $a Δ b$ and asks for the value of $-1Δ(1Δ-1)$, so evaluate the expression in the parenthesis first. $(1Δ-1) = (-1 -1)²/1²$, which simplifies to $ (-2)²/1 = 4.$ Now evaluate $-1Δ4. -1Δ4 = (4 - (-1))² / (-1)² $, which simplifies to $(5)² / 1 = 25$. The correction answer is $25.$
$\Delta$ should be defined by $a\Delta b = (b-a)^2/a^2$ by the explanation.
First calculate $1\Delta -1$, which is equal to $(-1-1))^2/1^2 = 4$.
Then calculate $-1\Delta 4$, which is equal to $(4-(-1))^2/(-1)^2 = 25$.