MCQMediumJEE 2024Derivatives of Functions

JEE Mathematics 2024 Question with Solution

Given f(x)=x3+x2f(1)+xf(2)+f(3)f(x) = x^3 + x^2 f'(1) + x f''(2) + f'''(3), xRx \in \mathbb{R}. Then f(10)f'(10) is equal to:

  • A

    202202

  • B

    200200

  • C

    204204

  • D

    206206

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: f(x)=x3+x2f(1)+xf(2)+f(3)f(x) = x^3 + x^2 f'(1) + x f''(2) + f'''(3)

Find: f(10)f'(10)

Differentiate the given function:

f(x)=3x2+2xf(1)+f(2)f'(x) = 3x^2 + 2x f'(1) + f''(2)

Differentiate again:

f(x)=6x+2f(1)f''(x) = 6x + 2f'(1)

And the third derivative is:

f(x)=6f'''(x) = 6

Now use the relations shown in the solution:

f(1)=5,f(2)=2,f(3)=6f'(1) = -5, \quad f''(2) = 2, \quad f'''(3) = 6

Substitute these values into the expression for f(x)f(x):

f(x)=x3+x2(5)+x(2)+6=x35x2+2x+6f(x) = x^3 + x^2(-5) + x(2) + 6 = x^3 - 5x^2 + 2x + 6

Differentiate this simplified form:

f(x)=3x210x+2f'(x) = 3x^2 - 10x + 2

Now evaluate at x=10x = 10:

f(10)=3(10)210(10)+2=300100+2=202f'(10) = 3(10)^2 - 10(10) + 2 = 300 - 100 + 2 = 202

Therefore, the correct option is A and f(10)=202f'(10) = 202.

Using derivative conditions explicitly

Given: f(x)=x3+x2f(1)+xf(2)+f(3)f(x) = x^3 + x^2 f'(1) + x f''(2) + f'''(3)

Find: f(10)f'(10)

From the given form,

f(x)=3x2+2xf(1)+f(2)f'(x) = 3x^2 + 2x f'(1) + f''(2)

so at x=1x=1,

f(1)=3+2f(1)+f(2)f'(1) = 3 + 2f'(1) + f''(2)

Using f(2)=2f''(2)=2 from the solution,

f(1)=3+2f(1)+2f'(1) = 3 + 2f'(1) + 2

which gives

f(1)=5f'(1) = -5

Also,

f(x)=6x+2f(1)f''(x) = 6x + 2f'(1)

so at x=2x=2,

f(2)=12+2f(1)f''(2) = 12 + 2f'(1)

Using f(1)=5f'(1)=-5,

f(2)=1210=2f''(2) = 12 - 10 = 2

This is consistent with the value used above.

Now compute

f(10)=3(10)2+2(10)(5)+2=300100+2=202f'(10) = 3(10)^2 + 2(10)(-5) + 2 = 300 - 100 + 2 = 202

Hence, f(10)=202f'(10) = 202, so the correct option is A.

Common mistakes

  • Students may treat f(1)f'(1), f(2)f''(2), and f(3)f'''(3) as functions of xx. They are constants because the derivatives are evaluated at fixed points. Differentiate them as constants, not as variable expressions.

  • A common error is to substitute values into f(x)f(x) before first identifying derivative relations carefully. First compute f(x)f'(x), f(x)f''(x), and f(x)f'''(x) correctly, then use the evaluated conditions.

  • Some students differentiate x2f(1)x^2 f'(1) incorrectly as 2x2f(1)2x^2 f'(1) or xf(1)x f'(1). Since f(1)f'(1) is a constant, the derivative is 2xf(1)2x f'(1).

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