SOLUTION: Can you please show the work on this for me? x=2 y=-4 the answer is -80 2x^2+3xy-4y^2 I can't get to -80 and not sure how to do the 3xy part? Thank you!

Algebra ->  Equations -> SOLUTION: Can you please show the work on this for me? x=2 y=-4 the answer is -80 2x^2+3xy-4y^2 I can't get to -80 and not sure how to do the 3xy part? Thank you!      Log On


   

Question 548697: Can you please show the work on this for me?
x=2
y=-4
the answer is -80
2x^2+3xy-4y^2
I can't get to -80 and not sure how to do the 3xy part?
Thank you!
Answer by bucky(2189) About Me  (Show Source):
You can put this solution on YOUR website!
You are asked to evaluate
.
2x%5E2%2B3xy-4y%5E2+
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when x = 2 and y = -4
.
Start by evaluating 2x%5E2
.
When x = 2 then 2x%5E2+=+2%2A2%5E2. This is equal to 2%2A2%2A2+=+8
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Next evaluate 3xy
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Substitute 2 for x and -4 for y to get:
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3xy+=+3%2A2%2A-4. The 3*2 = 6 and then the 6*-4 = -24
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Finally, evaluate 4y%5E2 when y = -4. Substitute -4 for y to get:
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4y%5E2+=+4%2A%28-4%29%5E2 and this becomes 4%2A%28-4%29%2A%28-4%29+=+64
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Now combine all three terms. Don't forget that this last one has a minus sign preceding it.
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2x%5E2%2B3xy-4y%5E2+=+8+%2B%28+-24%29+-+%2864%29
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Remove the parentheses and then sum the terms on the right side to get the answer:
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8+-+24+-+64
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Combine the +8 and -24 to get -16. Then add the -16 to the -64 to get -80.
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Hope this helps you to find where you were having trouble. In this case it could easily involve being confused by the signs.
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