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Sal evaluates 3x²-8x+7 when x=-2. Created by Sal Khan and Monterey Institute for Technology and Education.
Video transcript
Evaluate 3x^2-8x+7 when x=-2. So to evaluate this expression when x is equal to -2 everywhere that we see an x we just have to substitute it with a -2. So it would be 3 times, instead of x^2, it would be -2^2 minus 8 times... instead of 8 times x, 8 times -2 plus seven. And so what does this give us? So we have 3 times -2^2. Well, that's just positive 4. Negative times a negative is a positive. Minus 8 times -2 is -16. -16 plus 7. So 3 times 4 is 12. You're sub... from that you're subtracting -16. Subtracting -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 if you add 7 to 28, you get... you get 35.