
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
A stone arch bridge is parabolic in shape with a water surface width of 20 meters at normal water level. The apex of the arch is 4 meters above the water surface.

1. When the water level rises by \( h \) meters from the normal level, the width of the water surface under the bridge is \( d \) meters. Find the function that expresses \( d \) as a function of \( h \).

2. If the water depth under the bridge at normal water level is 2 meters, and to ensure safe navigation for passing ships the width of the water surface under the bridge must not be less than 18 meters, at what point does this condition affect navigation?

Present the answer in LaTex format: \boxed{Your answer}
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