If a sweatshirt normally costs $40 but it is marked down 20%. How much would it cost?
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The square of a prime number is not prime.
a) let x ∈ R, If x ∈ {prime numbers}, then ∉{prime numbers}
there says that if x is a real and x is in the set of the prime numbers, then the square of x isn't in the set of prime numbers.
b) Prove or disprove the statement.
ok, if x is a prime number, then x only can be divided by himself. Now is easy to see that = x*x can be divided by himself and x, then x*x is not a prime number, because can be divided by another number different than himself