Question 44 of 44advanced🔧 ApplyLong Answer5 marks

The houses of a row are numbered consecutively from 1 to 49. Show that there is a value of xx such that the sum of the numbers of the houses preceding the house numbered xx is equal to the sum of the numbers of the houses following it. Find this value of xx.

Correct Answer

The houses are numbered consecutively from 1 to 49, which forms a sequence of natural numbers. From the context, using the method of Gauss, the sum of the first n positive integers is given by the formula:

Sn=n(n+1)2S_n = \frac{n(n+1)}{2}

Let the house number be xx. The sum of the numbers of the houses preceding the house numbered xx is the sum of the first x−1x-1 natural numbers, denoted as Sx−1S_{x-1}. The sum of the numbers of the houses following it is the total sum S49S_{49} minus the sum up to xx, denoted as S49−SxS_{49} - S_x.

According to the problem, the sum of preceding numbers is equal to the sum of following numbers. Using the hint provided, we can write the equation as:

Sx−1=S49−SxS_{x-1} = S_{49} - S_x

Substituting the formula for the sum of natural numbers into the equation:

(x−1)x2=49×502−x(x+1)2\frac{(x-1)x}{2} = \frac{49 \times 50}{2} - \frac{x(x+1)}{2}

To solve, multiply the entire equation by 2 to eliminate the denominators:

(x−1)x=49×50−x(x+1)(x-1)x = 49 \times 50 - x(x+1) x2−x=2450−x2−xx^2 - x = 2450 - x^2 - x

Simplifying this, we get:

2x2=24502x^2 = 2450 x2=1225x^2 = 1225

Taking the square root of both sides, we find the value of x:

x=35x = 35

Thus, the house number is 35.

Exercise: EXERCISE 5.4 (Optional) | Q: 4 | (Page Page 23)
For More Understanding

Explanation

The solution utilizes the sum formula for the first nn natural numbers derived from the Gauss example in the text (S=n(n+1)2S = \frac{n(n+1)}{2}). It sets up the equation based on the specific hint provided in the context (Sx−1=S49−SxS_{x-1} = S_{49} - S_x) to equate the sum of preceding terms with the sum of following terms. Solving the resulting quadratic equation yields the correct house number.

Solution Steps

  1. Step 1: Identify the sequence of house numbers as natural numbers from 1 to 49 and apply the sum formula Sn=n(n+1)2S_n = \frac{n(n+1)}{2} derived from the Gauss method.

  2. Step 2: Set up the equation based on the problem condition and the provided hint: Sx−1=S49−SxS_{x-1} = S_{49} - S_x.

  3. Step 3: Substitute the sum formula into the equation: (x−1)x2=49×502−x(x+1)2\frac{(x-1)x}{2} = \frac{49 \times 50}{2} - \frac{x(x+1)}{2}.

  4. Step 4: Simplify the equation to x2−x=1225−x2−xx^2 - x = 1225 - x^2 - x, which reduces to 2x2=24502x^2 = 2450.

  5. Step 5: Solve for xx to get x2=1225x^2 = 1225, resulting in x=35x = 35.