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# Evaluating Polynomials

Reference > Mathematics > Algebra > Polynomials

We will sometimes talk about "evaluating" a polynomial. Evaluating a polynomial simply means plugging in a particular value for the variable (or particular values for the variables, in the case of a multivariate polynomial) and finding out the total value of the expression.

Example: Find the value of x2 +3x + 2 when x = 7.

Solution: 72 + 3(7) + 2 = 49 + 21 + 2 = 72.

Example: Find the value of 7x3 - x when x =2.

Solution: 7(23) - 2 = 7(8) - 2 = 56 - 2 = 54.

Example: Find the value of 2x + 3y - xy, when x = 5 and y = - 1.

Solution: 2(5) +3(-1) - 5(-1) = 10 - 3 + 5 = 12.

Evaluating polynomials also gives rise to the concept of polynomial equations:

Example: Find a value of x for which the polynomial x2 + 3x + 2 evaluates to 20.

Solution: We can turn this into a polynomial equation simply be writing x2 + 3x + 2 = 20. In general, when confronted with a polynomial equation (unless it is of degree 1) our goal is to get everything on one side, and zero on the other: x2 + 3x - 18 = 0. Thus, (x - 3)(x + 6) = 0, so x = 3 or x = -6.

## Questions

1.
Evaluate the polynomial x2 - 4 when x = 10
2.
Evaluate the polynomial x5 + 1 when x = 2
3.
Evaluate the polynomial x3 + 3x2 + 3x + 1 when x = 10
4.
Find values of x for which the polynomial x2 + 10x evaluates to 24.
5.
Find a value of x for which the polynomial x3 + x2 and the polynomial x3 + 2x - 1 have the same value.
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