SOLUTION: 2x2 + 3xy -4y2 when x=2 and y=4 i would like to know how they got (-80) if any one can break it down this is from a sample test for college online

Algebra ->  Inequalities -> SOLUTION: 2x2 + 3xy -4y2 when x=2 and y=4 i would like to know how they got (-80) if any one can break it down this is from a sample test for college online      Log On


   

Question 100792: 2x2 + 3xy -4y2 when x=2 and y=4 i would like to know how they got (-80)
if any one can break it down this is from a sample test for college online
Answer by bucky(2189) About Me  (Show Source):
You can put this solution on YOUR website!
Given:
.
2x%5E2+%2B+3xy+-4y%5E2
.
You are to evaluate this when x = 2 and y = 4
.
Go to the given function and substitute 2 wherever there is an x and substitute 4 wherever there
is a y. When you make those substitutions you get:
.
2%282%29%5E2+%2B+3%282%29%284%29-4%284%29%5E2
.
Notice that 2%5E2+=+4 and 4%5E2+=+16
.
Make these substitutions and the problem then becomes:
.
2%2A4+%2B+3%2A2%2A4+-+4%2A16
.
The 2 times the 4 is 8. And the 3 times 2 times 4 is 24. And the -4 times 16 is -64. Substituting
these values reduces the problem to:
.
8+%2B+24+-+64+
.
and these combine to give:
.
-32
.
You are correct. Their answer of -80 is in error. Are you sure that the sign of the middle
term of 3xy is positive? If it were negative your answer would become:
.
8+-+24+-+64
.
and that would give an answer of -80.