[ Measuring lengths of segments and measures of angles. Adjacent angles.] Unusual constructions (miscellaneous). $\quad]$

On a wooden ruler, three marks are made: 0, 7, and 11 centimeters. How can you measure with it a segment equal to: a) 8 cm $;$ b) 5 cm

#
a) Using divisions of 7 and 11, you can measure 4 cm. Doing this twice, you will get a segment of 8 cm.

b) First method. Knowing how to measure 8 and 7, you can measure 1 cm. Doing this 5 times, you will get 5 cm.

Second method. $5=3 \cdot 11-4 \cdot 7$, so it is enough to measure 3 times 11 cm and then 4 times 7 in the opposite direction.