Answer: No, a triangle cannot be formed with these side lengths.
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Step-by-step explanation:
Recall that the triangle inequality theorem says that for a triangle to be possible, the sum of any two sides must be larger than the third side.
With these given side lengths, we can see that 5+5 = 10 is not larger than 20, so a triangle is not possible here. I recommend you try it out with 3 slips of paper of these lengths. You should find that the two 5 cm sides are too short to be able to connect to the 20 cm side properly.