December 2006

## Bridging that gap: solution

In last issue's Outer space we asked you to find the equation describing the Clifton suspension bridge in Bristol, using the fact that it is spans 214 metres and is 21.3 metres high.

Recall that the bridge describes a parabola with equation *y = x ^{2}/a*, so our aim is to find the number

*a*. Now assuming that the lowest point of the bridge lies at the origin

*(0,0)*, we see that the two points

*(-107, 21.3)*and

*(107, 21.3)*lie on the parabola. This gives:

*21.3 = 107*and so

^{2}/a*a = 107*

^{2}/21.3 = 537.5Here are the parabola and the bridge:

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