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Inverse Trigonometric Functions question

2023 · 15 Apr · Shift 1 · Q28
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Inverse Trigonometric Functions question

2023 · 15 Apr · Shift 1 · Q28

JEE MainMathematicsInverse Trigonometric FunctionsMCQ+4 / −1
If the domain of the function f(x)=log⁡e(4x2+11x+6)+sin⁡−1(4x+3)+cos⁡−1(10x+63)f(x)=\log _{e}\left(4 x^{2}+11 x+6\right)+\sin ^{-1}(4 x+3)+\cos ^{-1}\left(\frac{10 x+6}{3}\right)f(x)=loge​(4x2+11x+6)+sin−1(4x+3)+cos−1(310x+6​) is (α,β](\alpha, \beta](α,β], then 36∣α+β∣36|\alpha+\beta|36∣α+β∣ is equal to :
  1. A
    72
  2. B
    54
  3. C
    45
  4. D
    63
View written solutionFree

Correct answer: C

  1. Given function

f(x)=ln⁡(4x2+11x+6)+sin⁡−1(4x+3)+cos⁡−1(10x+63)f(x)=\ln(4x^2+11x+6)+\sin^{-1}(4x+3)+\cos^{-1}\left(\frac{10x+6}{3}\right)f(x)=ln(4x2+11x+6)+sin−1(4x+3)+cos−1(310x+6​)

We need the domain of this function.

For the domain, all three terms must be defined simultaneously.


  1. Condition from the logarithm

For ln⁡(4x2+11x+6)\ln(4x^2+11x+6)ln(4x2+11x+6) to be defined,

4x2+11x+6>04x^2+11x+6>04x2+11x+6>0

Factorize:

4x2+11x+6=(4x+3)(x+2)4x^2+11x+6=(4x+3)(x+2)4x2+11x+6=(4x+3)(x+2)

So we need

(4x+3)(x+2)>0(4x+3)(x+2)>0(4x+3)(x+2)>0

Critical points are x=−2x=-2x=−2 and x=−34x=-\frac34x=−43​.

Thus,

x∈(−∞,−2)∪(−34,∞)x\in(-\infty,-2)\cup\left(-\frac34,\infty\right)x∈(−∞,−2)∪(−43​,∞)


  1. Condition from sin⁡−1(4x+3)\sin^{-1}(4x+3)sin−1(4x+3)

For sin⁡−1(4x+3)\sin^{-1}(4x+3)sin−1(4x+3) to be defined,

−1≤4x+3≤1-1\le 4x+3\le 1−1≤4x+3≤1

Subtract 333:

−4≤4x≤−2-4\le 4x\le -2−4≤4x≤−2

Divide by 444:

−1≤x≤−12-1\le x\le -\frac12−1≤x≤−21​


  1. Condition from cos⁡−1(10x+63)\cos^{-1}\left(\frac{10x+6}{3}\right)cos−1(310x+6​)

For cos⁡−1(y)\cos^{-1}(y)cos−1(y) to be defined,

−1≤y≤1-1\le y\le 1−1≤y≤1

So,

−1≤10x+63≤1-1\le \frac{10x+6}{3}\le 1−1≤310x+6​≤1

Multiply by 333:

−3≤10x+6≤3-3\le 10x+6\le 3−3≤10x+6≤3

Subtract 666:

−9≤10x≤−3-9\le 10x\le -3−9≤10x≤−3

Divide by 101010:

−910≤x≤−310-\frac9{10}\le x\le -\frac3{10}−109​≤x≤−103​


  1. Intersect all conditions

We need

x∈[−1,−12]∩[−910,−310]∩[(−∞,−2)∪(−34,∞)]x\in\left[ -1,-\frac12\right] \cap \left[-\frac9{10},-\frac3{10}\right] \cap \left[(-\infty,-2)\cup\left(-\frac34,\infty\right)\right]x∈[−1,−21​]∩[−109​,−103​]∩[(−∞,−2)∪(−43​,∞)]

First intersect the inverse trigonometric conditions:

[−1,−12]∩[−910,−310]=[−910,−12]\left[ -1,-\frac12\right] \cap \left[-\frac9{10},-\frac3{10}\right]=\left[-\frac9{10},-\frac12\right][−1,−21​]∩[−109​,−103​]=[−109​,−21​]

Now intersect with the logarithm condition:

[−910,−12]∩[(−∞,−2)∪(−34,∞)]\left[-\frac9{10},-\frac12\right] \cap \left[(-\infty,-2)\cup\left(-\frac34,\infty\right)\right][−109​,−21​]∩[(−∞,−2)∪(−43​,∞)]

Since [−910,−12]\left[-\frac9{10},-\frac12\right][−109​,−21​] does not meet (−∞,−2)(-\infty,-2)(−∞,−2), only the second part matters:

[−910,−12]∩(−34,∞)=(−34,−12]\left[-\frac9{10},-\frac12\right] \cap \left(-\frac34,\infty\right)=\left(-\frac34,-\frac12\right][−109​,−21​]∩(−43​,∞)=(−43​,−21​]

So the domain is

(α,β]=(−34,−12](\alpha,\beta]=\left(-\frac34,-\frac12\right](α,β]=(−43​,−21​]

Hence,

α=−34,β=−12\alpha=-\frac34,\qquad \beta=-\frac12α=−43​,β=−21​


  1. Compute 36∣α+β∣36|\alpha+\beta|36∣α+β∣

α+β=−34−12=−34−24=−54\alpha+\beta=-\frac34-\frac12=-\frac34-\frac24=-\frac54α+β=−43​−21​=−43​−42​=−45​

Therefore,

∣α+β∣=54|\alpha+\beta|=\frac54∣α+β∣=45​

So,

36∣α+β∣=36⋅54=9⋅5=4536|\alpha+\beta|=36\cdot \frac54=9\cdot 5=4536∣α+β∣=36⋅45​=9⋅5=45


  1. Option check

The value is

45\boxed{45}45​

So the correct option is C.

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