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BBA-DBE Bangalore 2025 PYQsQuantitative Ability. Free, no login required.

If f: R -> R is defined by f(x) = 2x – 3 and g: R -> R is defined by g(x) = x³ + 5, then (fog)⁻¹(-9) is equal to:

Solution

✅ Correct Option: 1

Given functions f(x)=2x−3f(x) = 2x - 3 and g(x)=x3+5g(x) = x^3 + 5.

First, find the composite function (fog)(x)(fog)(x):

(fog)(x)=f(g(x))=f(x3+5)(fog)(x) = f(g(x)) = f(x^3 + 5)

Substitute x3+5x^3+5 into f(x)f(x): 2(x3+5)−32(x^3 + 5) - 3

(fog)(x)=2x3+10−3=2x3+7(fog)(x) = 2x^3 + 10 - 3 = 2x^3 + 7

Now, we need to find (fog)−1(−9)(fog)^{-1}(-9).

Let (fog)−1(−9)=k(fog)^{-1}(-9) = k. By definition of inverse functions, this means (fog)(k)=−9(fog)(k) = -9.

Using the expression for (fog)(x)(fog)(x) we found:

2k3+7=−92k^3 + 7 = -9

Subtract 7 from both sides: 2k3=−9−72k^3 = -9 - 7

2k3=−162k^3 = -16

Divide by 2: k3=−8k^3 = -8

Take the cube root of both sides: k=−83k = \sqrt[3]{-8}

k=−2k = -2

Therefore, (fog)−1(−9)=−2(fog)^{-1}(-9) = -2.

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