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Differentiation question

2024 · 9 Apr · Shift 1 · Q33
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  5. /2024 · 9 Apr · Shift 1 · Q33

Differentiation question

2024 · 9 Apr · Shift 1 · Q33

JEE MainMathematicsDifferentiationMCQ+4 / −1
Let f(x)=ax3+bx2+cx+41f(x)=a x^3+b x^2+c x+41f(x)=ax3+bx2+cx+41 be such that f(1)=40,f′(1)=2f(1)=40, f^{\prime}(1)=2f(1)=40,f′(1)=2 and f′′(1)=4f^{\prime \prime}(1)=4f′′(1)=4. Then a2+b2+c2a^2+b^2+c^2a2+b2+c2 is equal to:
  1. A
    54
  2. B
    51
  3. C
    73
  4. D
    62
View written solutionFree

Correct answer: B

  1. Given function f(x)=ax3+bx2+cx+41f(x)=ax^3+bx^2+cx+41f(x)=ax3+bx2+cx+41

  2. Use the condition f(1)=40f(1)=40f(1)=40 a+b+c+41=40a+b+c+41=40a+b+c+41=40 a+b+c=−1...(1)a+b+c=-1 \quad ...(1)a+b+c=−1...(1)

  3. Find the first derivative f′(x)=3ax2+2bx+cf'(x)=3ax^2+2bx+cf′(x)=3ax2+2bx+c Given f′(1)=2f'(1)=2f′(1)=2, so 3a+2b+c=2...(2)3a+2b+c=2 \quad ...(2)3a+2b+c=2...(2)

  4. Find the second derivative f′′(x)=6ax+2bf''(x)=6ax+2bf′′(x)=6ax+2b Given f′′(1)=4f''(1)=4f′′(1)=4, so 6a+2b=46a+2b=46a+2b=4 3a+b=2...(3)3a+b=2 \quad ...(3)3a+b=2...(3)

  5. Solve the system of equations

    From (3): b=2−3ab=2-3ab=2−3a

    Substitute into (2): 3a+2(2−3a)+c=23a+2(2-3a)+c=23a+2(2−3a)+c=2 3a+4−6a+c=23a+4-6a+c=23a+4−6a+c=2 −3a+c=−2-3a+c=-2−3a+c=−2 c=3a−2c=3a-2c=3a−2

    Now substitute bbb and ccc into (1): a+(2−3a)+(3a−2)=−1a+(2-3a)+(3a-2)=-1a+(2−3a)+(3a−2)=−1 a−3a+3a+2−2=−1a-3a+3a+2-2=-1a−3a+3a+2−2=−1 a=−1a=-1a=−1

    Then, b=2−3(−1)=5b=2-3(-1)=5b=2−3(−1)=5 c=3(−1)−2=−5c=3(-1)-2=-5c=3(−1)−2=−5

  6. Compute a2+b2+c2a^2+b^2+c^2a2+b2+c2 a2+b2+c2=(−1)2+52+(−5)2a^2+b^2+c^2=(-1)^2+5^2+(-5)^2a2+b2+c2=(−1)2+52+(−5)2 =1+25+25=51=1+25+25=51=1+25+25=51

  7. Match with options The correct option is: B: 51\boxed{\text{B: }51}B: 51​

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