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In calculus, integration by substitution, also known as u-substitution, reverse chain rule or change of variables, [1] is a method for evaluating integrals and antiderivatives. It is the counterpart to the chain rule for differentiation , and can loosely be thought of as using the chain rule "backwards."
The substitution is described in most integral calculus textbooks since the late 19th century, usually without any special name. [5] It is known in Russia as the universal trigonometric substitution , [ 6 ] and also known by variant names such as half-tangent substitution or half-angle substitution .
Change of variables is an operation that is related to substitution. However these are different operations, as can be seen when considering differentiation or integration (integration by substitution). A very simple example of a useful variable change can be seen in the problem of finding the roots of the sixth-degree polynomial:
The simplest method for solving a system of linear equations is to repeatedly eliminate variables. This method can be described as follows: In the first equation, solve for one of the variables in terms of the others. Substitute this expression into the remaining equations. This yields a system of equations with one fewer equation and unknown.
Advice on the application of change of variable to PDEs is given by mathematician J. Michael Steele: [1] "There is nothing particularly difficult about changing variables and transforming one equation to another, but there is an element of tedium and complexity that slows us down.
A substitution σ is called a linear substitution if tσ is a linear term for some (and hence every) linear term t containing precisely the variables of σ ' s domain, i.e. with vars(t) = dom(σ). A substitution σ is called a flat substitution if xσ is a variable for every variable x. A substitution σ is called a renaming substitution if it ...
Vieta's substitution is a method ... three in the auxiliary variables, the main difference being that Lagrange's method explains why these auxiliary variables appear ...
Integration by substitution – Technique in integral evaluation; Leibniz integral rule – Differentiation under the integral sign formula; Product rule – Formula for the derivative of a product; Quotient rule – Formula for the derivative of a ratio of functions; Triple product rule – Relation between relative derivatives of three variables