Problem 7 - A Complete Resource Book in Mathematics for JEE Main 2017

Question

\(10^{n}+3.4^{n+2}+k\) is divisible by 9 for \(n \in N\). Then, the least positive integral value of \(k\) is (A) 1 (B) 3 (C) 5 (D) 7

Step-by-Step Solution

Answer

The least positive integral value of \(k\) is 5.

Step 1: Determine divisibility rule for 9

To determine divisibility by 9, the sum of digits of a number in base 10 must be divisible by 9. We will need the expression to be appropriate such that it can be divisible by 9.

Step 2: Analyze individual terms modulo 9

We need each part of the expression \(10^n + 3 \cdot 4^{n+2} + k\) to contribute appropriately modulo 9. Since \(10 \equiv 1 \pmod{9}\), we have \(10^n \equiv 1^n \equiv 1 \pmod{9}\).

Step 3: Simplify and express 4 to a power

Consider the term \(4^{n+2}\): \(4 \equiv 4 \pmod{9}\), so \(4^{n+2} \equiv 4^{n+2} \pmod{9}\). Because \(4^2 \equiv 16 \equiv 7 \equiv 1 \pmod{9}\), simplifying gives \(4^{n+2} \equiv 4^n \equiv 4 \pmod{9}\).

Step 4: Sum and find suitable k

Substitute the values obtained: \(1 + 3 \times 4 + k \equiv 1 + 12 + k \equiv 13 + k \pmod{9}\). We need this to be divisible by 9. Therefore, \(13 + k \equiv 0 \pmod{9}\), which implies \(k \equiv -13 \equiv -4 \equiv 5 \pmod{9}\).

Step 5: Verify the result

The calculation shows that \(k \equiv 5 \pmod{9}\) ensures that \(10^n + 3 \cdot 4^{n+2} + k\) is divisible by 9. Verifying by substitution, \(13 + 5 = 18\), which is divisible by 9.

Key Concepts

Modular Arithmetic

Modular arithmetic is like clockwork math, where numbers wrap around at a certain point known as the modulus. Imagine a clock with numbers from 0 to 8; whenever you reach 9, you circle back to 0. This simple model illustrates a modulus of 9.

Here's how it works in our problem: To check divisibility, we say a number is congruent to another if they leave the same remainder when divided by 9. This is written as \(a \equiv b \pmod{9}\). In our exercise, for example, we see that \(10 \equiv 1 \pmod{9}\). This equivalence tells us 10 leaves the same remainder as 1 when divided by 9.

Using this, terms like \(10^n\) become much simpler. Since \(10 \equiv 1 \pmod{9}\), \(10^n \equiv 1^n \equiv 1 \pmod{9}\). You'll find modular arithmetic incredibly helpful for simplifying large computations and ensuring terms fit neatly within constraints, like divisibility rules.

Power Simplification

When dealing with expressions raised to a power, like \(4^{n+2}\), it's useful to simplify by recognizing patterns in the results of repeated modular operations.

For instance, powers of 4 when viewed through the lens of modulus 9 behave as follows:

• \(4^1 \equiv 4 \pmod{9}\)
• \(4^2 \equiv 16 \equiv 7 \pmod{9}\)
• \(4^3 \equiv 28 \equiv 1 \pmod{9}\)

A pattern appears: multiples of three for the exponent return to 1 modulo 9 (\(4^3 \equiv 1 \pmod{9}\)). Understanding this cycle, powers can be flattened, allowing for easier calculation. \(4^{n+2}\) becomes easy to reduce: it breaks into components \(4^n \cdot 4^2\) which resolves back into results we know, \(4^{n+2} \equiv 4 \cdot 1 \equiv 4 \pmod{9}\).

These cyclic simplifications can greatly reduce time spent on tedious calculations, turning a complex expression into a simple modular equivalent.

Integer Value Determination

In number theory, determining the appropriate integer value for an equation to hold a particular property, like divisibility, is crucial. In this exercise, the task was to choose the smallest positive integer value of \(k\) such that \(10^n + 3 \cdot 4^{n+2} + k\) becomes divisible by 9.

After analyzing each part of the expression under modulo 9 and simplifying, the expression simplifies to \(13 + k \pmod{9}\). The job now is to find \(k\) so that \(13 + k \equiv 0 \pmod{9}\), which simply means \(13 + k\) is a multiple of 9.

Breaking this down, \(-13 \equiv -4 \), and since \(-4 + k \equiv 0 \pmod{9}\), we solve for \(k\equiv 4 + 9n\) (where \(n\) is a whole number). Choosing \(n = 1\) gives \(k = 5\), the smallest such integer. By substituting \(k = 5\), the sum becomes 18, proving its divisibility by 9.

This practical application shows how breaking a problem into its modular components allows for step-by-step integer resolution, ensuring conforming to exact divisibility standards.