Which statements are true?
1. 20 is the greatest common factor of 40 and 80.
2. 64 is a perfect square.
3. x(x^2+1)=x^3+x
4. x^2-81 cannot be factored
5. 3x^3 is the GCF of 15x^4+9x^2+3
6. (x+5) is a factor of x^2-2x+15
7. (4x^2+2)(x^2-2)=4x^4-6x^2-4
8. x+3 is a factor of x^2+9
9. The product of two binomials is always a trinomial.
10. 57 is a prime number.