Question:
Make the correct alternative in the following question:
For all $n \in \mathbf{N}, 3 \times 5^{2 n+1}+2^{3 n+1}$ is divisible by
(a) 19
(b) 17
(c) 23
(d) 25
Solution:
Let $\mathrm{P}(n)=3 \times 5^{2 n+1}+2^{3 n+1}$, for all $n \in \mathbf{N}$.
For $n=1$,
$\mathrm{P}(1)=3 \times 5^{2+1}+2^{3+1}$
$=3 \times 5^{3}+2^{4}$
$=375+16$
$=391$
$=17 \times 23$
For $n=2$,
$\mathrm{P}(2)=3 \times 5^{4+1}+2^{6+1}$
$=3 \times 5^{5}+2^{7}$
$=9375+128$
$=9503$
$=17 \times 13 \times 43$
As, $\operatorname{HCF}(391,9503)=17$
So, $\mathrm{P}(n)$ is divisible by 17 .
Hence, the correct alternative is option (b).
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- JEE Main
- Exam Pattern
- Previous Year Papers
- PYQ Chapterwise
- Physics
- Kinematics 1D
- Kinemetics 2D
- Friction
- Work, Power, Energy
- Centre of Mass and Collision
- Rotational Dynamics
- Gravitation
- Calorimetry
- Elasticity
- Thermal Expansion
- Heat Transfer
- Kinetic Theory of Gases
- Thermodynamics
- Simple Harmonic Motion
- Wave on String
- Sound waves
- Fluid Mechanics
- Electrostatics
- Current Electricity
- Capacitor
- Magnetism and Matter
- Electromagnetic Induction
- Atomic Structure
- Dual Nature of Matter
- Nuclear Physics
- Radioactivity
- Semiconductors
- Communication System
- Error in Measurement & instruments
- Alternating Current
- Electromagnetic Waves
- Wave Optics
- X-Rays
- All Subjects
- Physics
- Motion in a Plane
- Law of Motion
- Work, Energy and Power
- Systems of Particles and Rotational Motion
- Gravitation
- Mechanical Properties of Solids
- Mechanical Properties of Fluids
- Thermal Properties of matter
- Thermodynamics
- Kinetic Theory
- Oscillations
- Waves
- Electric Charge and Fields
- Electrostatic Potential and Capacitance
- Current Electricity
- Thermoelectric Effects of Electric Current
- Heating Effects of Electric Current
- Moving Charges and Magnetism
- Magnetism and Matter
- Electromagnetic Induction
- Alternating Current
- Electromagnetic Wave
- Ray Optics and Optical Instruments
- Wave Optics
- Dual Nature of Radiation and Matter
- Atoms
- Nuclei
- Semiconductor Electronics: Materials, Devices and Simple Circuits.
- Chemical Effects of Electric Current,
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