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Hyperbola question

2022 · 28 Jun · Shift 2 · Q34
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  5. /2022 · 28 Jun · Shift 2 · Q34

Hyperbola question

2022 · 28 Jun · Shift 2 · Q34

JEE MainMathematicsHyperbolaMCQ+4 / −1
Let a > 0, b > 0. Let e and l respectively be the eccentricity and length of the latus rectum of the hyperbola x2a2−y2b2=1{{{x^2}} \over {{a^2}}} - {{{y^2}} \over {{b^2}}} = 1a2x2​−b2y2​=1. Let e' and l' respectively be the eccentricity and length of the latus rectum of its conjugate hyperbola. If e2=1114l{e^2} = {{11} \over {14}}le2=1411​l and (e′)2=118l′{\left( {e'} \right)^2} = {{11} \over 8}l'(e′)2=811​l′, then the value of 77a+44b77a + 44b77a+44b is equal to :
  1. A
    100
  2. B
    110
  3. C
    120
  4. D
    130
View written solutionFree

Correct answer: D

  1. Given hyperbola

x2a2−y2b2=1\frac{x^2}{a^2}-\frac{y^2}{b^2}=1a2x2​−b2y2​=1

For this hyperbola:

  • Eccentricity: e=1+b2a2=a2+b2ae=\sqrt{1+\frac{b^2}{a^2}}=\frac{\sqrt{a^2+b^2}}{a}e=1+a2b2​​=aa2+b2​​
  • Length of latus rectum: l=2b2al=\frac{2b^2}{a}l=a2b2​

The condition given is e2=1114le^2=\frac{11}{14}le2=1411​l

Now, e2=1+b2a2=a2+b2a2e^2=1+\frac{b^2}{a^2}=\frac{a^2+b^2}{a^2}e2=1+a2b2​=a2a2+b2​

and 1114l=1114⋅2b2a=11b27a\frac{11}{14}l=\frac{11}{14}\cdot \frac{2b^2}{a}=\frac{11b^2}{7a}1411​l=1411​⋅a2b2​=7a11b2​

So, a2+b2a2=11b27a\frac{a^2+b^2}{a^2}=\frac{11b^2}{7a}a2a2+b2​=7a11b2​

Multiply by 7a27a^27a2: 7(a2+b2)=11ab27(a^2+b^2)=11ab^27(a2+b2)=11ab2

So we get 7a2+7b2=11ab2(1)7a^2+7b^2=11ab^2 \qquad (1)7a2+7b2=11ab2(1)


  1. Conjugate hyperbola

The conjugate hyperbola is y2b2−x2a2=1\frac{y^2}{b^2}-\frac{x^2}{a^2}=1b2y2​−a2x2​=1

For this hyperbola:

  • Transverse semi-axis =b=b=b
  • Conjugate semi-axis =a=a=a

Hence,

  • Eccentricity: e′=1+a2b2=a2+b2be'=\sqrt{1+\frac{a^2}{b^2}}=\frac{\sqrt{a^2+b^2}}{b}e′=1+b2a2​​=ba2+b2​​
  • Length of latus rectum: l′=2a2bl'=\frac{2a^2}{b}l′=b2a2​

Given: (e′)2=118l′(e')^2=\frac{11}{8}l'(e′)2=811​l′

Now, (e′)2=1+a2b2=a2+b2b2(e')^2=1+\frac{a^2}{b^2}=\frac{a^2+b^2}{b^2}(e′)2=1+b2a2​=b2a2+b2​

and 118l′=118⋅2a2b=11a24b\frac{11}{8}l'=\frac{11}{8}\cdot \frac{2a^2}{b}=\frac{11a^2}{4b}811​l′=811​⋅b2a2​=4b11a2​

Thus, a2+b2b2=11a24b\frac{a^2+b^2}{b^2}=\frac{11a^2}{4b}b2a2+b2​=4b11a2​

Multiply by 4b24b^24b2: 4(a2+b2)=11a2b4(a^2+b^2)=11a^2b4(a2+b2)=11a2b

So, 4a2+4b2=11a2b(2)4a^2+4b^2=11a^2b \qquad (2)4a2+4b2=11a2b(2)


  1. Solve the system

From (1): 7(a2+b2)=11ab27(a^2+b^2)=11ab^27(a2+b2)=11ab2

From (2): 4(a2+b2)=11a2b4(a^2+b^2)=11a^2b4(a2+b2)=11a2b

Let S=a2+b2S=a^2+b^2S=a2+b2

Then, 7S=11ab2(3)7S=11ab^2 \qquad (3)7S=11ab2(3) 4S=11a2b(4)4S=11a^2b \qquad (4)4S=11a2b(4)

Divide (3) by (4): 74=11ab211a2b=ba\frac{7}{4}=\frac{11ab^2}{11a^2b}=\frac{b}{a}47​=11a2b11ab2​=ab​

Hence, b=7a4b=\frac{7a}{4}b=47a​

Substitute into (4): 4(a2+b2)=11a2b4(a^2+b^2)=11a^2b4(a2+b2)=11a2b

Using b=7a4b=\frac{7a}{4}b=47a​, a2+b2=a2+49a216=65a216a^2+b^2=a^2+\frac{49a^2}{16}=\frac{65a^2}{16}a2+b2=a2+1649a2​=1665a2​

So, 4⋅65a216=11a2⋅7a44\cdot \frac{65a^2}{16}=11a^2\cdot \frac{7a}{4}4⋅1665a2​=11a2⋅47a​

65a24=77a34\frac{65a^2}{4}=\frac{77a^3}{4}465a2​=477a3​

Multiply by 4: 65a2=77a365a^2=77a^365a2=77a3

Since a>0a>0a>0, a=6577a=\frac{65}{77}a=7765​

Then, b=74⋅6577=6544b=\frac{7}{4}\cdot \frac{65}{77}=\frac{65}{44}b=47​⋅7765​=4465​


  1. Compute the required value

77a+44b=77⋅6577+44⋅654477a+44b=77\cdot \frac{65}{77}+44\cdot \frac{65}{44}77a+44b=77⋅7765​+44⋅4465​

=65+65=130=65+65=130=65+65=130


  1. Final answer

The value of 77a+44b77a+44b77a+44b is

130\boxed{130}130​

So the correct option is D.

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