8th Grade - Evaluating Functions


  • Evaluating a function fx means finding the value of the function that corresponds to a given input value of x.

Evaluating functions

  • The process of evaluating a function is described in the following two steps mentioned below:
    • Replace the input variable with the provided value
    • Simplify the expression to get the final result
  • Sometimes a function can also be expressed in tabular form or graphical form. We find the output value in such cases by looking at the table or graph.

Solved Examples

Question 1: Given fx=2x2-5x+1, find the value of f3.

Solution: fx=2x2-5x+1=2×32-5×3+1=2×9-15+1=18-15+1=4


Question 2: Based on the table below, find the value of f7.

Solution: From the given table, we get f7=-5.


Question 3: Find f-1 from the graph shown below.

Solution: From the given graph, we get f-1=-4.

Cheat Sheet

  • To evaluate a function at a given input, we substitute the variable in the function with the given input value and then simplify it to get the final result.

Blunder Areas

  • It is recommended to put the substituted values inside parentheses in order to avoid committing mistakes.
  • Note that fx+afx+fa.