Answer:
John should use a protractor to measure an angle of 70°.
Explanation:
70° is an angle for which there is no construction procedure.
Angles of measures 15°, 18°, 30°, 36°, and 45° can be constructed, along with their supplements, complements, and values multiplied by any positive integer or divided by any power of 2. Of course, the sums and differences of any of these angles can be constructed. (The more steps in the construction, the greater the loss of accuracy.)
Because there is no construction procedure for an angle of 70°, John must create the angle using a measuring tool. A protractor is the tool of choice.
John should use a protractor to measure an angle of 70° relative to one end of his 5 cm baseline.
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Comment on John's technique
It is likely John would create a more accurate representation of the desired triangle by drawing the 7 cm segment first. This would give a longer baseline for use with the protractor, and would mean the length through the point marked using the protractor would not have to be extrapolated as far.