Our **substitution** feature allows you to answer the question "What *would* this expression, such as \(3+A\), represent if we substituted a different value for \(A\)?" without ever changing the original value of \(A\).

## Substitution within a Function

The substitution function, represented by the command ‘with,’ replaces a variable in an expression or function with a constant value, another variable, or a list.

Where would the parabola \(f\left(x\right)=ax^{2}+bx+c\) move if \(b\) was replaced * with * \(-1\)? Model \(f\left(x\right)\) after substituting \(b\) * with * \(-1\) to visualize the change.

Where would the polygon be if it was rotated by \(90\) degrees instead of \(45\) degrees? Model the rotation * with * an angle of \(90\) degrees.

## Substitution within the Same Expression Line

You can evaluate expressions by substituting variables with constant values in the same line. You can also substitute a variable with another expression, but if that expression depends on a free variable, it will need to be defined separately.

## Learn More

Please write in with any questions or feedback to support@desmos.com.