Set, Relations and FunctionsMTP Apr 21Question 1978 of 217
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Let R\displaystyle R be the set of real numbers such that the function f:R→R\displaystyle f: R \to R and g:R→R\displaystyle g: R \to R are defined by f(x)=x2+3x+1\displaystyle f(x) = x^2+3x+1 and g(x)=2x−3\displaystyle g(x) = 2x-3. Find (fog)\displaystyle (fog).

Options

A4x2+6x+1\displaystyle 4x^2+6x+1
Bx2−6x+1\displaystyle x^2-6x+1
C4x2−6x+1\displaystyle 4x^2-6x+1
Dx2+6x+1\displaystyle x^2+6x+1
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Correct Answer

✅ Option b — x2−6x+1\displaystyle x^2-6x+1

All Options:

  • A4x2+6x+1\displaystyle 4x^2+6x+1
  • Bx2−6x+1\displaystyle x^2-6x+1
  • C4x2−6x+1\displaystyle 4x^2-6x+1
  • Dx2+6x+1\displaystyle x^2+6x+1

Detailed Solution & Explanation

To find the composite function (f∘g)(x)=f(g(x))\displaystyle (f \circ g)(x) = f(g(x)), we substitute the expression for g(x)\displaystyle g(x) into f(x)\displaystyle f(x):
Given:
f(x)=x2+3x+1f(x) = x^2 + 3x + 1
g(x)=2x−3g(x) = 2x - 3

Substitute g(x)\displaystyle g(x) in place of x\displaystyle x in the function f(x)\displaystyle f(x):
f(g(x))=(2x−3)2+3(2x−3)+1f(g(x)) = (2x - 3)^2 + 3(2x - 3) + 1

**Step 1: Expand (2x−3)2\displaystyle (2x - 3)^2** using the identity (a−b)2=a2−2ab+b2\displaystyle (a-b)^2 = a^2 - 2ab + b^2:
(2x−3)2=4x2−12x+9(2x - 3)^2 = 4x^2 - 12x + 9

**Step 2: Expand 3(2x−3)\displaystyle 3(2x - 3)**:
3(2x−3)=6x−93(2x - 3) = 6x - 9

**Step 3: Combine all terms together**:
f(g(x))=(4x2−12x+9)+(6x−9)+1f(g(x)) = (4x^2 - 12x + 9) + (6x - 9) + 1
f(g(x))=4x2+(−12x+6x)+(9−9+1)f(g(x)) = 4x^2 + (-12x + 6x) + (9 - 9 + 1)
f(g(x))=4x2−6x+1f(g(x)) = 4x^2 - 6x + 1

**Discrepancy & Typographical Error:**
Mathematically, the composite function simplifies to 4x2−6x+1\displaystyle 4x^2 - 6x + 1, which is **Option C**. However, the answer key for this specific mock paper (MTP Apr 21) has a typographical error, designating **Option B** (x2−6x+1\displaystyle x^2 - 6x + 1) as the correct answer. The mathematically correct result is 4x2−6x+1\displaystyle 4x^2 - 6x + 1, as confirmed in identical questions in other test papers (e.g. MTP June 24).

Hence, **Option B** is the correct answer (according to the textbook key), but the mathematically proven correct value is 4x2−6x+1\displaystyle 4x^2 - 6x + 1 (**Option C**).

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