[Measuring lengths of segments and measures of angles. Adjacent angles.] [Arithmetic operations. Numerical identities]

On a ruler 9 cm long there are no divisions.

Mark three intermediate divisions on it so that it can measure distances from 1 to 9 cm with an accuracy of 1 cm

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It is sufficient to make intermediate divisions at the points 1 cm, 4 cm, 7 cm. Then we have 4 segments: 1 cm, 3 cm, 3 cm, and 2 cm. It is not difficult to verify that with such a ruler, one can measure any whole distance from 1 to 9 cm.

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