About this video
Sal evaluates the polynomial 3x^2 - 8x + 7 when x = -2. He substitutes -2 for every x, squares the negative number carefully, and handles subtracting a negative to get 35.
You'll be able to
- substitute a value for x into a polynomial.
- square a negative number correctly.
- simplify an expression that subtracts a negative number.
Key words
- evaluate
- Find the value of an expression when the variable is replaced by a number.
- substitute
- Replace a variable with a given number everywhere it appears.
Transcript
Evaluate 3x squared minus 8x plus 7 when x is equal to negative 2. So to evaluate this expression when x is equal to negative 2, everywhere that we see an x, we just have to substitute it with a negative 2. So it would be 3 times, instead of x squared, it would be 3 times negative 2 squared minus 8 times-- instead of 8 times x, 8 times negative 2 plus 7.
And so what does this give us? So we have 3 times negative 2 squared. Well, that's just positive 4. A negative times a negative is a positive. Minus 8 times negative 2 is negative 16 plus 7. So 3 times 4 is 12. From that, you're subtracting negative 16. Subtracting negative 16 is the same thing as adding positive 16.
So it's 12 plus 16 plus 7. And so this is equal to 12 plus 16 is 28. And then if you add 7 to 28, you get 35.
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