It is known that the two sides of an isosceles triangle are 3cm and 8cm, respectively______ ．

If 3cm is the waist length, 8cm is the bottom length, ∵ 3 + 3 = 6 ＜ 8, it can't form a triangle, ∵ which is not suitable for the problem, and it is omitted; if 3cm is the bottom length, 8cm is the waist length, then the circumference of the triangle is: 3 + 8 + 8 = 19 (CM). So the answer is: 19cm

It is known that the circumference of an isosceles triangle is 29 cm and one side is 5 cm

1. If 5cm is waist length, the third side length is 29-5-5 = 9cm
2. If 5cm is not waist length, waist length is (29-5) / 2 = 12cm

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