Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Quadratic Equation and Inequalities question

2021 · 31 Aug · Shift 2 · Q30
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Quadratic Equation and Inequalities
  5. /2021 · 31 Aug · Shift 2 · Q30

Quadratic Equation and Inequalities question

2021 · 31 Aug · Shift 2 · Q30

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
The sum of the roots of the equation x+1−2log⁡2(3+2x)+2log⁡4(10−2−x)=0x + 1 - 2{\log _2}(3 + {2^x}) + 2{\log _4}(10 - {2^{ - x}}) = 0x+1−2log2​(3+2x)+2log4​(10−2−x)=0, is :
  1. A
    log2 14
  2. B
    log2 11
  3. C
    log2 12
  4. D
    log2 13
View written solutionFree

Correct answer: B

  1. Given equation

x+1−2log⁡2(3+2x)+2log⁡4(10−2−x)=0x + 1 - 2\log_2(3+2^x) + 2\log_4(10-2^{-x})=0x+1−2log2​(3+2x)+2log4​(10−2−x)=0

We need the sum of all roots.


  1. Convert everything to base 2

Recall:

log⁡4a=log⁡2alog⁡24=12log⁡2a\log_4 a = \frac{\log_2 a}{\log_2 4} = \frac{1}{2}\log_2 alog4​a=log2​4log2​a​=21​log2​a

So,

2log⁡4(10−2−x)=2⋅12log⁡2(10−2−x)=log⁡2(10−2−x)2\log_4(10-2^{-x}) = 2\cdot \frac{1}{2}\log_2(10-2^{-x}) = \log_2(10-2^{-x})2log4​(10−2−x)=2⋅21​log2​(10−2−x)=log2​(10−2−x)

Hence the equation becomes

x+1−2log⁡2(3+2x)+log⁡2(10−2−x)=0x+1-2\log_2(3+2^x)+\log_2(10-2^{-x})=0x+1−2log2​(3+2x)+log2​(10−2−x)=0

Also,

x=log⁡2(2x),1=log⁡22x = \log_2(2^x), \qquad 1=\log_2 2x=log2​(2x),1=log2​2

Thus

x+1=log⁡2(2x)+log⁡22=log⁡2(2x+1)x+1 = \log_2(2^x)+\log_2 2 = \log_2(2^{x+1})x+1=log2​(2x)+log2​2=log2​(2x+1)

Therefore,

log⁡2(2x+1)+log⁡2(10−2−x)−2log⁡2(3+2x)=0\log_2(2^{x+1}) + \log_2(10-2^{-x}) - 2\log_2(3+2^x)=0log2​(2x+1)+log2​(10−2−x)−2log2​(3+2x)=0

Using log laws,

log⁡2(2x+1(10−2−x)(3+2x)2)=0\log_2\left(\frac{2^{x+1}(10-2^{-x})}{(3+2^x)^2}\right)=0log2​((3+2x)22x+1(10−2−x)​)=0

So,

2x+1(10−2−x)(3+2x)2=1\frac{2^{x+1}(10-2^{-x})}{(3+2^x)^2}=1(3+2x)22x+1(10−2−x)​=1

That is,

2x+1(10−2−x)=(3+2x)22^{x+1}(10-2^{-x})=(3+2^x)^22x+1(10−2−x)=(3+2x)2


  1. Substitute t=2xt=2^xt=2x

Since 2x>02^x>02x>0, let

t=2x⇒2−x=1tt=2^x \quad \Rightarrow \quad 2^{-x}=\frac{1}{t}t=2x⇒2−x=t1​

Then the equation becomes

2t(10−1t)=(3+t)22t\left(10-\frac{1}{t}\right)=(3+t)^22t(10−t1​)=(3+t)2

Simplify:

20t−2=t2+6t+920t-2=t^2+6t+920t−2=t2+6t+9

t2−14t+11=0t^2-14t+11=0t2−14t+11=0


  1. Solve the quadratic

t2−14t+11=0t^2-14t+11=0t2−14t+11=0

Using the quadratic formula,

t=14±196−442=14±1522=7±38t=\frac{14\pm \sqrt{196-44}}{2} = \frac{14\pm \sqrt{152}}{2}=7\pm \sqrt{38}t=214±196−44​​=214±152​​=7±38​

So the two possible values are

t1=7+38,t2=7−38t_1=7+\sqrt{38}, \qquad t_2=7-\sqrt{38}t1​=7+38​,t2​=7−38​

Both are positive, so both are valid since t=2x>0t=2^x>0t=2x>0.

Thus roots are

x1=log⁡2(7+38),x2=log⁡2(7−38)x_1=\log_2(7+\sqrt{38}), \qquad x_2=\log_2(7-\sqrt{38})x1​=log2​(7+38​),x2​=log2​(7−38​)


  1. Find the sum of roots

x1+x2=log⁡2(7+38)+log⁡2(7−38)x_1+x_2 = \log_2(7+\sqrt{38})+\log_2(7-\sqrt{38})x1​+x2​=log2​(7+38​)+log2​(7−38​)

=log⁡2((7+38)(7−38))=\log_2\left((7+\sqrt{38})(7-\sqrt{38})\right)=log2​((7+38​)(7−38​))

=log⁡2(49−38)=log⁡211=\log_2(49-38)=\log_2 11=log2​(49−38)=log2​11


  1. Match with options

log⁡211\boxed{\log_2 11}log2​11​

So the correct option is B.


  1. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

They agree.

PreviousNext

More from Quadratic Equation and Inequalities

  • Let α and β be the roots of the equation 5x2 + 6x – 2 = 0. If Sn =α n + β n, n = 1, 2, 3...., then :2020 · MCQ
  • Let f(x) be a quadratic polynomial such that f(–1) + f(2) = 0. If one of the roots of f(x) = 0 is 3, then its other root lies in :2020 · MCQ
  • If α and β are the roots of the equation x2 + px + 2 = 0 and α1​ and β1​ are the roots of the equation 2x2 + 2qx + 1 = 0, then (α−α1​)(β−β1​)(α+β1​)(β+α1​)…2020 · MCQ
  • The set of all real values of λ for which the quadratic equations, (λ 2 + 1)x2 – 4 λ x + 2 = 0 always have exactly one root in the interval (0, 1) is :2020 · MCQ
  • Let α and β be the roots of x2 - 3x + p=0 and γ and δ be the roots of x2 - 6x + q = 0. If α,β,γ,δ form a geometric progression.Then ratio (2q + p) : (2q - p) is:2020 · MCQ
  • Let [t] denote the greatest integer ≤ t. Then the equation in x, [x]2 + 2[x+2] - 7 = 0 has :2020 · MCQ
  • Let λe0 be in R. If α and β are the roots of the equation, x2 - x + 2 λ= 0 and α and γ are the roots of the equation, 3x2−10x+27λ=0, then λβγ​ is…2020 · MCQ
  • The product of the roots of the equation 9x2 - 18|x| + 5 = 0 is :2020 · MCQ