Science, Maths & Technology

### Become an OU student Succeed with maths: part 2

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# 3.5 Combining formulas

There is a quicker way to the answer which requires two of the formula to be combined to give a new formula for relative stride length.

The formula for relative stride length was:

It is known that:

This means that it is possible to substitute the formula for hip height in the formula for relative stride length. This then gives:

This new formula could be used directly with the information that was given in the question. This version would also make more sense in the field, as it contains the two pieces of information that would be known on site.

In the final activity for this week you’ll consider why it is important to always think about whether the answer you have obtained looks sensible. This is something that you should always ask yourself. It can act as a prompt to check over your working or logic in any problem.

## Activity 11 Relative stride length: what’s wrong?

A student used the formula to calculate the relative stride length in the footprints activity.

The student entered the key sequence on their calculator for the calculation as: 104.6 ÷ 4 × 21.8. This gave an answer of approximately 570, so the student concluded that the person was probably running very fast. Can you explain where the student made the mistake and why they should have been suspicious of their answer?