# Need to calculate: The value of F(3) for the function F(x)=begin{cases}2x if x<3 -5x if xgeq 3 end{cases}

Need to calculate: The value of F(3) for the function

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Formula used:
Some functions are defined by different equations for various parts for their domains.
Those functions are called piecewise-defined functions.
To solve the piecewise-defined function for an input, we will determine that which part of the domain it belongs to and use the appropriate formula for that part of the domain.
Calculation:
Consider the provided function,

The function provided is defined in multiple equations.
Therefore, to determine F(3) identify in which equation 3 will lie.
3 lies in the interval $x\ge 3$. Therefore, to determine the value of F (3), $F\left(x\right)=-5x$ will be used.
Substitute $x=3$ into the equation $F\left(x\right)=-5x$.
$F\left(x\right)=-5x$
$F\left(-1\right)=5\ast \left(3\right)=-15$
Hence, the value of F(3) is -15.