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JEE Main
Exam
JEE Main
Subject
Mathematics
Chapter
Differentiation

JEE Main · Mathematics · Differentiation

A cubic polynomial curve y = f(x) = ax^3 + bx^2 + cx + d has a...

A cubic polynomial curve y=f(x)=ax3+bx2+cx+dy = f(x) = ax^3 + bx^2 + cx + d has a point of inflection at x=1x = 1 and passes through the point (1,1)(1, 1). If the tangent to the curve at the point (0,2)(0, 2) is given by the line y=3x+2y = 3x + 2, then the value of a2+b2+c2+d2a^2 + b^2 + c^2 + d^2 is equal to:

Options

  1. A53
  2. B119
  3. C29
  4. D23

Correct answer

A. 53

Step-by-step solution

Let f(x)=ax3+bx2+cx+df(x) = ax^3 + bx^2 + cx + d.

Since the tangent to the curve at (0,2)(0, 2) is y=3x+2y = 3x + 2, the curve passes through (0,2)(0, 2).
Thus, f(0)=2⇒d=2f(0) = 2 \Rightarrow d = 2.

The slope of the tangent at x=0x = 0 is 33.
f′(x)=3ax2+2bx+cf'(x) = 3ax^2 + 2bx + c
f′(0)=3⇒c=3f'(0) = 3 \Rightarrow c = 3.

The curve has a point of inflection at x=1x = 1, so f′′(1)=0f''(1) = 0.
f′′(x)=6ax+2bf''(x) = 6ax + 2b
f′′(1)=6a+2b=0⇒3a+b=0⇒b=−3af''(1) = 6a + 2b = 0 \Rightarrow 3a + b = 0 \Rightarrow b = -3a.

The curve passes through (1,1)(1, 1), so f(1)=1f(1) = 1.
a+b+c+d=1a + b + c + d = 1
a+(−3a)+3+2=1a + (-3a) + 3 + 2 = 1
−2a+5=1⇒−2a=−4⇒a=2-2a + 5 = 1 \Rightarrow -2a = -4 \Rightarrow a = 2.

Then b=−3(2)=−6b = -3(2) = -6.

We have a=2a = 2, b=−6b = -6, c=3c = 3, d=2d = 2.
Therefore, a2+b2+c2+d2=22+(−6)2+32+22=4+36+9+4=53a^2 + b^2 + c^2 + d^2 = 2^2 + (-6)^2 + 3^2 + 2^2 = 4 + 36 + 9 + 4 = 53.

Answer: 53
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