Given the line AB = 60cm, draw the line BC on the line AB so that BC = 20cm, point D is the midpoint of AC, and find the length of CD
As shown in the figure, (1) when point C is on the line AB, CD = 12 (AB − BC) = 12 (60 − 20) = 12 × 40 = 20 (CM); (2) when point C is on the extension line of line AB, CD = 12 (AB + BC) = 12 (60 + 20) = 12 × 80 = 40 (CM);; the length of CD is 20cm or 40cm
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