Class 11 RD Sharma Solutions – Chapter 29 Limits – Exercise 29.6 | Set 1

Question 1. Lim_x→∞{(3x – 1)(4x – 2)}/{(x + 8)(x – 1)}.

Solution:

We have,

Lim_x→∞{(3x – 1)(4x – 2)}/{(x + 8)(x – 1)}

= 

= 

When x → ∞, (1/x) → 0.

= (3 × 4)/(1 × 1)

= 12

Question 2. Lim_x→∞{(3x³ – 4x² + 6x – 1)}/{(2x³ + x² – 5x + 7)}.

Solution:

We have,

Lim_x→∞{(3x³ – 4x² + 6x – 1)}/{(2x³ + x² – 5x + 7)}

=

=

When x → ∞, (1/x), (1/x²), (1/x³) → 0.

= 3/2

Question 3. Lim_x→∞{(5x³ – 6)}/{√(9 + 4x⁶)}.

Solution:

We have,

Lim_x→∞{(5x³ – 6)}/{√(9 + 4x⁶)}

=

= 

When x → ∞, (1/x), (1/x³) → 0.

= 5/√4

= 5/2

Question 4. Lim_x→∞{√(x² + cx) – x}

Solution:

We have,

Lim_x→∞{√(x²+cx)-x}

On rationalizing numerator, we get

= Lim_x→∞{(x² + cx) – x²}/{√(x² + cx) + x}

= Lim_x→∞(cx)/{√(x² + cx) + x}

= Lim_x→∞(cx)/[x{√(x + c/x) + 1}]

= Lim_x→∞(c)/{√(1 + c/x) + 1}

When x → ∞, (1/x) → 0.

= c/(√1 + 1)

= c/2

Question 5. Lim_x→∞{√(x + 1) – √x}

Solution:

We have,

Lim_x→∞{√(x + 1) – √x}

On rationalizing numerator, we get

= Lim_x→∞{(x+1)-x}/{√(x+1)+√x}

= Lim_x→∞(1)/{√(x+1)+√x}

= 

When x → ∞, (1/x) → 0.

= 0

Question 6. Lim_x→∞{√(x² + 7x) – x}

Solution:

We have,

Lim_x→∞{√(x² + 7x) – x}

On rationalizing numerator, we get

= Lim_x→∞{(x²+7x)-x2}/{√(x²+7x)+x}

= Limx→∞(7x)/{√(x²+7x)+x}

=

=

When x → ∞, (1/x) → 0.

= 7/(√1 + 1)

= 7/2

Question 7.  Lim_x→∞(x)/{√(4x² + 1) – 1}

Solution:

We have,

Lim_x→∞(x)/{√(4x² + 1) – 1}

Rationalising denominator.

= Lim_x→∞[x{√(4x² + 1) + 1}]/{(4x² + 1) – 1}

= Lim_x→∞[x{√(4x² + 1) + 1}]/(4x²)

= Lim_x→∞[{√(4x² + 1) + 1}]/(4x)

=

When x → ∞, (1/x²) → 0.

= √4/4

= 2/4

= 1/2

Question 8. Lim_n→∞(n²)/{1 + 2 + 3 + 4 + ……………. + n}

Solution:

We have,

Lim_n→∞(n²)/{1 + 2 + 3 + 4 + ……………. + n}

=

= Lim_n→∞(2n)/(n+1)

= Lim_n→∞(2)/(1+1/n)

When n → ∞, (1/n) → 0

= 2/(1 + 0)

= 2

Question 9. Lim_x→∞(3x^-1 + 4x^-2)/(5x^-1 + 6x^-2)

Solution:

We have,

Lim_x→∞(3x^-1 + 4x^-2)/(5x^-1 + 6x^-2)

= 

= 

When x → ∞, (1/x) → 0.

= 3/5

Question 10. Lim_x→∞{√(x² + a²) – √(x² + b²)}/{√(x² + c²) – √(x² + d²)}

Solution:

We have,

 Lim_x→∞{√(x² + a²) – √(x² + b²)}/{√(x² + c²) – √(x² + d²)}

On rationalizing numerator and denominator, we get

=

=

=

=

When x → ∞, (1/x²) → 0.

=

= (a² – b²)/(c² – d²)

Question 11. Lim_n→∞{(n + 2)! + (n + 1)!}/{(n + 2)! – (n + 1)!}.

Solution:

We have,

Lim_n→∞{(n + 2)! + (n + 1)!}/{(n + 2)! – (n + 1)!}

= Lim_n→∞{(n + 2)(n + 1)! + (n + 1)!}/{(n + 2)(n + 1)! – (n + 1)!}

= Lim_n→∞[(n + 1)!{(n + 2) + 1}]/[(n + 1)!{(n + 2) – 1}]

= Lim_n→∞(n + 3)/(n + 1)

= Lim_n→∞[n(1 + 3/n)]/[n(1 + 1/n)]

When n → ∞, (1/n) → 0.

= 1/1

= 1

Question 12. Lim_x→∞[x{√(x² + 1) – √(x² – 1)}]

Solution:

We have,

Lim_x→∞[x{√(x² + 1) – √(x² – 1)}]

On rationalizing numerator, we get

= Lim_x→∞[x{(x² + 1) – (x² – 1)}]/{√(x² + 1) + √(x² – 1)}

= Lim_x→∞(2x)/{√(x² + 1) + √(x² – 1)}

= Lim_x→∞(2x)/[x{√(1 + 1/x²) + √(1 – 1/x²)}]

= Lim_x→∞(2)/[{√(1 + 1/x²) + √(1 – 1/x²)}]

When x → ∞, (1/x²) → 0.

= 2/(√1 + √1)

= 2/2

= 1

Question 13.  Lim_x→∞[√(x + 2){√(x + 1) – √x}]

Solution:

We have,

 Lim_x→∞[√(x + 2){√(x + 1) – √x}]

On rationalizing numerator, we get

= Lim_x→∞[√(x + 2){(x + 1) – x}]/{√(x + 1) + √x}

= Lim_x→∞[√(x + 2)]/{√(x + 1) + √x}

= Lim_x→∞[x√(1 + 2/x)]/[x{√(1 + 1/x) + √1}]

= Lim_x→∞[√(1 + 2/x)]/{√(1 + 1/x) + √1}

When x → ∞, (1/x) → 0.

= 1/(√1 + √1)

= 1/2