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In mathematics, LHS is informal shorthand for the left-hand side of an equation. Similarly, RHS is the right-hand side. The two sides are in practice interchangeable, since equality is symmetric. This abbreviation is seldom if ever used in print; it is very informal.

More generally, these terms may apply to an inequation or inequality. In the inequality case, there need not be symmetry. The right-hand side is everything on the right side of a test operator in an expression. Conversely, the left-hand side is everything on the left side.

Some examples

The expression on the right side of the "=" sign is the right side of the equation and the expression on the left of the "=" is the left side of the equation.

For example, in

x + 5 = y + 8,

"x + 5" is the left-hand side (LHS) and "y + 8" is the right-hand side (RHS).
Homogeneous and inhomogeneous equations

In solving mathematical equations, particularly linear simultaneous equations, differential equations and integral equations, the terminology homogeneous is often used for equations with the RHS set equal to zero. The corresponding inhomogeneous or nonhomogeneous equation then has the RHS with some given data, but of a general character.

The typical case is of some operator L, with the difference being that between the equation

Lf = 0,

to be solved for a function f, and the equation

Lf = g,

with g a fixed function, to solve again for f. The point of the terminology appears for L a linear operator. Then any solution of the inhomogeneous equation may have a solution of the homogeneous equation added to it, and still remain a solution.

For example in mathematical physics, the homogeneous equation may correspond to a physical theory formulated in empty space, while the inhomogeneous equation asks for more 'realistic' solutions with some matter, or charged particles.
Syntax

More abstractly, when using infix notation

T*U

the term T stands as the left-hand side and U as the right-hand side of the operator *. This usage is less common, though.
See also

equal sign

Undergraduate Texts in Mathematics

Graduate Texts in Mathematics

Graduate Studies in Mathematics

Mathematics Encyclopedia

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