Since prime numbers only have two factors (itself and 1) then it would have to be the same number. So it would be the number besides one. Ex. Prime number: 7, the GCF would be 7 if it was compared with itself.
Answer:

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1. sqrt(98) = 7 sqrt(2)
2. sqrt(y^6) = y^3
3. sqrt(a^7) = a^7/2
4. sqrt(12x^3y^2) = 2xy sqrt(3x)
5. sqrt(36x^2y^4) = 6xy^2
6. sqrt(48ab^3) = 4b sqrt(3ab)
7. sqrt(10a^5b^2) = a^2b sqrt(10a)
8. sqrt(20x^3y^10 = 2xy^5 sqrt(5x)