Question: User: A builder rested a $3$ m ladder against a vertical wall so that the foot of the ladder was $70$ cm away from the base of the wall.
The ladder slipped down the wall, causing the foot of the ladder to move $50$ cm further away from the base of the wall.
How far down the wall, in centimetres, did the ladder slip? Give your answer to the nearest $1$ cm.
1. **State the problem:**
A ladder of length $3$ m (which is $300$ cm) is resting against a vertical wall. Initially, the foot of the ladder is $70$ cm from the wall. The ladder slips, moving the foot $50$ cm further away, so the new distance from the wall is $70 + 50 = 120$ cm.
We need to find how far down the wall the ladder slipped, i.e., the vertical distance the top of the ladder moved down.
2. **Set up the initial situation:**
The ladder, wall, and ground form a right triangle with the ladder as the hypotenuse.
Initial vertical height $h_1$ satisfies:
$$
300^2 = 70^2 + h_1^2
$$
3. **Calculate initial height $h_1$:**
$$
300^2 = 90000
$$
$$
70^2 = 4900
$$
$$
h_1^2 = 90000 - 4900 = 85100
$$
$$
h_1 = \sqrt{85100} \approx 291.7 \text{ cm}
$$
4. **Set up the situation after slipping:**
The foot is now $120$ cm from the wall, ladder length remains $300$ cm.
New height $h_2$ satisfies:
$$
300^2 = 120^2 + h_2^2
$$
Calculate $h_2$:
$$
120^2 = 14400
$$
$$
h_2^2 = 90000 - 14400 = 75600
$$
$$
h_2 = \sqrt{75600} \approx 274.9 \text{ cm}
$$
5. **Calculate how far the ladder slipped down the wall:**
$$
\text{Slip distance} = h_1 - h_2 = 291.7 - 274.9 = 16.8 \text{ cm}
$$
6. **Final answer rounded to nearest cm:**
$$
17 \text{ cm}
$$
**Answer:** The ladder slipped down the wall approximately $17$ cm.