The complex numbers are a field.
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The square root of a negative number is not real. However, there is a field of numbers known as the complex number field which contains the reals and in which negative numbers have square roots. Complex numbers can all be expressed in the form a+bi where a and b are real and i is the pure imaginary such that i2=1. Please see the related links for more information about complex and imaginary numbers.
A "complex number" is a number of the form a+bi, where a and b are both real numbers and i is the principal square root of -1. Since b can be equal to 0, you see that the real numbers are a subset of the complex numbers. Similarly, since a can be zero, the imaginary numbers are a subset of the complex numbers. So let's take two complex numbers: a+bi and c+di (where a, b, c, and d are real). We add them together and we get: (a+c) + (b+d)i The sum of two real numbers is always real, so a+c is a real number and b+d is a real number, so the sum of two complex numbers is a complex number. What you may really be wondering is whether the sum of two non-real complex numbers can ever be a real number. The answer is yes: (3+2i) + (5-2i) = 8. In fact, the complex numbers form an algebraic field. The sum, difference, product, and quotient of any two complex numbers (except division by 0) is a complex number (keeping in mind the special case that both real and imaginary numbers are a subset of the complex numbers).
You can, but the process is slightly complicated, because addition in the Complex field is like vector addition. If z1 = (r1, a1), and If z2 = (r2, a2) Then, if z = (r, a) r = sqrt(r12 + r22) and a = arctan[(r1sina1 + r2sina2)/(r1cosa1 + r2cosa2)]
The set of real numbers, R, is a mathematical field. For any three real numbers x, y and z and the operations of addition and multiplication,• x + y belongs to R (closure under addition)• (x + y) + z = x + (y + z) (associative property of addition)• There is an element, 0, in R, such that x + 0 = 0 + x = x (existence of additive identity)• There is an element, -x, in R, such that x + (-x) = (-x) + x = 0 (existence of additive inverse)• x + y = y + x (Abelian or commutative property of addition)• x * y belongs to R (closure under multiplication)• (x * y) * z = x * (y * z) (associative property of multiplication)• There is an element, 1, in R, such that x * 1 = 1 * x = x (existence of multiplicative identity)• For every non-zero x, there is an element, 1/x, in R, such that x * 1/x = 1/x * x = 1 (existence of multiplicative inverse)• x * (y + z) = x*y + x * z (distributive property of multiplication over addition)
One significant feature of complex numbers is that all polynomial equations of order n, in the complex field, have n solutions. When multiple roots are Given any set of complex numbers {a(0),  … , a(n)}, such that at least one of a(1) to a(n) is non-zero, the equation a(n)*z^n + a(n-1)*z^(n-1) + ... + a(0) has at least one solution in the complex field. This is the Fundamental Theorem of Algebra and establishes the set of Complex numbers as a closed field. [a(0), ... , a(n) should be written with suffices but this browser has decided not to be cooperative!] The above solution is the complex root of the equation. In fact, if the equation is of order n, that is, if the coefficient a(n) is non-zero then, taking account of the multiplicity, the equation has exactly n roots (some of which may be real).