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To solve mathematical problems with Symbolic Math Toolbox™, define symbolic objects to represent various mathematical objects. This example discusses the usage of the following symbolic objects:

symbolic numbers

symbolic scalar variables, functions, and expressions

symbolic equations

symbolic vectors and matrices

symbolic matrix variables

*(since R2021a)*

Defining a number as a symbolic number instructs MATLAB^{®} to treat the number as an exact form instead of using a numeric
approximation. For example, use a symbolic number to represent the argument of an
inverse trigonometric function $$\theta ={\mathrm{sin}}^{-1}(1/\sqrt{2})$$.

Create the symbolic number $$\text{1/}\sqrt{2}$$ using `sym`

, and assign it to
`a`

.

a = sym(1/sqrt(2))

a = 2^(1/2)/2

Find the inverse sine of `a`

. The result is the symbolic number
`pi/4`

.

thetaSym = asin(a)

thetaSym = pi/4

You can convert a symbolic number to variable-precision arithmetic by using `vpa`

. The result is a decimal number with 32 significant
digits.

thetaVpa = vpa(thetaSym)

thetaVpa = 0.78539816339744830961566084581988

To convert the symbolic number to a double-precision number, use
`double`

. For more information about whether to use numeric or
symbolic arithmetic, see Choose Numeric or Symbolic Arithmetic.

thetaDouble = double(thetaSym)

thetaDouble = 0.7854

Defining variables, functions, and expressions as symbolic objects enables you to
perform algebraic operations with those symbolic objects, including simplifying formulas
and solving equations. For example, use a symbolic scalar variable, function, and
expression to represent the quadratic function $$f(x)={x}^{2}+x-2$$. For brevity, a symbolic scalar variable is also called a
*symbolic variable*.

Create a symbolic scalar variable `x`

using `syms`

. You can also use `sym`

to create a symbolic scalar variable. For more information about
whether to use `syms`

or `sym`

, see Choose syms or sym Function. Define a
symbolic expression `x^2 + x - 2`

to represent the right side of the
quadratic equation and assign it to `f(x)`

. The identifier
`f(x)`

now refers to a symbolic function that represents the
quadratic
function.

syms x f(x) = x^2 + x - 2

f(x) = x^2 + x -2

You can then evaluate the quadratic function by providing its input argument inside
the parentheses. For example, evaluate
`f(2)`

.

fVal = f(2)

fVal = 4

You can also solve the quadratic equation $$f(x)=0$$. Use `solve`

to find the roots of the quadratic
equation. `solve`

returns the two solutions as a vector of two symbolic
numbers.

sols = solve(f)

sols = -2 1

Defining a mathematical equation as a symbolic equation enables you to find the solution of the equation. For example, use a symbolic equation to solve the trigonometric problem $$2\mathrm{sin}(t)\mathrm{cos}(t)=1$$.

Create a symbolic function `g(t)`

using `syms`

.
Assign the symbolic expression `2*sin(t)*cos(t)`

to
`g(t)`

.

syms g(t) g(t) = 2*sin(t)*cos(t)

g(t) = 2*cos(t)*sin(t)

`==`

operator and assign the mathematical
relation `g(t) == 1`

to `eqn`

. The identifier
`eqn`

is a symbolic equation that represents the trigonometric
problem.eqn = g(t) == 1

eqn = 2*cos(t)*sin(t) == 1

Use `solve`

to find the solution of the trigonometric
problem.

sol = solve(eqn)

sol = pi/4

Use a symbolic vector and matrix to represent and solve a system of linear equations.

$$\begin{array}{c}x+2y=u\\ 4x+5y=v\end{array}$$

You can represent the system of equations as a vector of two symbolic equations. You
can also represent the system of equations as a matrix problem involving a matrix of
symbolic numbers and a vector of symbolic variables. For brevity, any vector of symbolic
objects is called a *symbolic vector* and any matrix of symbolic
objects is called a *symbolic matrix*.

Create two symbolic equations `eq1`

and `eq2`

.
Combine the two equations into a symbolic
vector.

syms u v x y eq1 = x + 2*y == u; eq2 = 4*x + 5*y == v; eqns = [eq1, eq2]

eqns = [x + 2*y == u, 4*x + 5*y == v]

Use `solve`

to find the solutions of the system of equations
represented by `eqns`

. `solve`

returns a structure
`S`

with fields named after each of the variables in the equations.
You can access the solutions using dot notation, as `S.x`

and
`S.y`

.

S = solve(eqns); S.x

ans = (2*v)/3 - (5*u)/3

S.y

ans = (4*u)/3 - v/3

Another way to solve the system of linear equations is to convert it to matrix form.
Use `equationsToMatrix`

to convert the system of equations to matrix
form and assign the output to `A`

and `b`

. Here,
`A`

is a symbolic matrix and `b`

is a symbolic
vector. Solve the matrix problem by using the matrix division \
operator.

[A,b] = equationsToMatrix(eqns,x,y)

A = [1, 2] [4, 5] b = u v

sols = A\b

sols = (2*v)/3 - (5*u)/3 (4*u)/3 - v/3

*Since R2021a*

Use symbolic matrix variables to evaluate differentials with respect to vectors.

$$\begin{array}{l}\alpha ={y}^{\text{T}}Ax\\ \frac{\partial \alpha}{\partial x}={y}^{\text{T}}A\\ \frac{\partial \alpha}{\partial y}={x}^{\text{T}}{A}^{\text{T}}\end{array}$$

Symbolic matrix variables represent matrices, vectors, and scalars as atomic symbols. Symbolic matrix variables offer a concise display in typeset and show mathematical formulas with more clarity. You can take vector-based expressions from textbooks and enter them in Symbolic Math Toolbox.

Create three symbolic matrix variables `x`

, `y`

, and
`A`

using the `syms`

command with the
`matrix`

argument. Nonscalar symbolic matrix variables are
displayed as bold characters in the Command Window and in the Live
Editor.

syms x [4 1] matrix syms y [3 1] matrix syms A [3 4] matrix x y A

x =xy =yA =A

`alpha`

. Find the differential of `alpha`

with
respect to the vectors `x`

and
`y`

.alpha = y.'*A*x

alpha =y.'*A*x

diff(alpha,x)

ans =y.'*A

diff(alpha,y)

alpha =x.'*A.'

This table compares various symbolic objects that are available in Symbolic Math Toolbox.

Symbolic Objects | Examples of MATLAB Commands | Size of Symbolic Objects | Data Type |
---|---|---|---|

symbolic number |
a = 1/sqrt(sym(2)) theta = asin(a) a = 2^(1/2)/2 theta = pi/4 | `1` -by-`1` | `sym` |

symbolic scalar variable |
syms x y u v | `1` -by-`1` | `sym` |

symbolic function |
syms x f(x) = x^2 + x - 2 syms g(t) g(t) = 2*sin(t)*cos(t) f(x) = x^2 + x - 2 g(t) = 2*cos(t)*sin(t) | `1` -by-`1` | `symfun` |

symbolic equation |
syms u v x y eq1 = x + 2*y == u eq2 = 4*x + 5*y == v eq1 = x + 2*y == u eq2 = 4*x + 5*y == v | `1` -by-`1` | `sym` |

symbolic expression |
syms x expr = x^2 + x - 2 expr2 = 2*sin(x)*cos(x) expr = x^2 + x - 2 expr2 = 2*cos(x)*sin(x) | `1` -by-`1` | `sym` |

symbolic vector |
syms u v b = [u v] b = [u, v] | `1` -by-`n` or
`m` -by-`1` | `sym` |

symbolic matrix |
syms A x y A = [x y; x*y y^2] A = [ x, y] [x*y, y^2] | `m` -by-`n` | `sym` |

symbolic multidimensional array |
syms A [2 1 2] A A(:,:,1) = A1_1 A2_1 A(:,:,2) = A1_2 A2_2 | `sz1` -by-`sz2` -...-`szn` | `sym` |

symbolic matrix variable (since R2021a) |
syms A B [2 3] matrix A B
| `m` -by-`n` | `symmatrix` |

`sym`

| `symfun`

| `syms`

| `symmatrix`

| `str2sym`

| `symmatrix2sym`