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Algebra Level 4

The sum of two irrational numbers is always irrational.

The sum of two rational numbers is always rational.

The sum of two complex numbers is always complex.

Of the 3 statements given above, how many of these statements are true?

3 2 0 1

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1 solution

展豪 張
Apr 27, 2016

The first is wrong, consider ( 1 + 2 ) + ( 2 ) = 1 (1+\sqrt 2)+(-\sqrt 2)=1
The second is correct.
( a , c Z , b , d N ) , a b + c d = a d + b c b d Q (a,c \in \mathbb Z,b,d \in \mathbb N), \dfrac ab + \dfrac cd = \dfrac {ad+bc}{bd} \in \mathbb Q
The third is correct.
( a , b , c , d R ) , ( a + b i ) + ( c + d i ) = ( a + c ) + ( b + d ) i C (a,b,c,d \in \mathbb R), (a+bi)+(c+di)=(a+c)+(b+d)i \in \mathbb C
Note that the difference between the first statement and the third statement is that even a pure real number (imaginary part = 0) is a complex number.

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