To solve the problem shown in the image, we can proceed step by step. The problem involves a bridge with two straight sections, each inclined at an angle and meeting at a point above the horizontal plane. We need to find the distance $d$ between the closest endpoints of the two sections.

Step 1: Understand the problem

• The bridge has two straight sections, each inclined such that they form a gradient (slope) of 0.8.
• The height from the horizontal to the peak where the two sections meet is 6 meters.
• We are asked to find the horizontal distance $d$ between the closest endpoints of these sections.

Step 2: Set up the mathematical model

• Let the length of each section of the bridge be $L$.
• The gradient is given by the ratio of the vertical rise to the horizontal run, i.e., the slope $m = \text{gradient} = \frac{\text{vertical rise}}{\text{horizontal run}}$.

Given that the gradient is 0.8 and the vertical height (rise) is 6 meters:

$\text{Gradient} = 0.8 = \frac{\text{vertical rise}}{\text{horizontal run}} = \frac{6 \text{ meters}}{\text{horizontal run}}$

So, the horizontal run (half of $d$) can be calculated as:

$\text{Horizontal run} = \frac{6 \text{ meters}}{0.8} = 7.5 \text{ meters}$

Step 3: Calculate the horizontal distance $d$

• Since the distance between the closest end points of the two sections is the horizontal distance between the points where the bridge sections touch the ground:

  $d = 2 \times \text{horizontal run} = 2 \times 7.5 = 15 \text{ meters}$

Step 4: Correction

To get $d = 4$ meters, there seems to be an additional constraint or a mistake in the model setup.

Key Insight:

Since the problem explicitly asks for $d = 4$ meters, let's use the setup provided and calculate again. It appears the given answer might involve the specific geometry or constraints that need to be considered differently.

Correct Calculation:

• Given the height $h = 6$ meters and slope $m = 0.8$, the horizontal run per side should be:

$\text{Horizontal run} = \frac{6}{0.8} = 7.5 \text{ meters per side.}$

• The distance $d$ between the two closest points is the sum of the horizontal distances each section runs from the center point.

However, since the correct answer is $d = 4$ meters, this implies that each section might span less horizontally, leading to a smaller value than initially calculated. Specifically, there might be a horizontal overlap due to symmetry, which affects the $d$ value.

Thus, the correct value could come from symmetry or re-evaluation of the geometry with more accurate constraints. Please check the assumptions or recalibrate the gradient for such adjustments.

Final Steps:

• Verify that the constraints and assumptions in the problem lead to the correct geometric interpretation, resulting in $d = 4$ meters under a different setup.

Would you like a deeper exploration into the geometry or adjustments to the gradient to explain $d = 4$ meters?

Related Questions:

1. What happens to $d$ if the gradient changes?
2. How do changes in height affect the calculation of $d$?
3. What alternative methods exist to verify the result?
4. How is the concept of symmetry applied in such problems?
5. How would the problem change if only one section of the bridge were given?

Tip:

Always double-check geometric constraints and assumptions when the expected result seems off, as subtle details can lead to different interpretations.