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

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) (Show Source): You can put this solution on YOUR website!
Given:
.

.
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:
.

.
Notice that and
.
Make these substitutions and the problem then becomes:
.

.
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:
.

.
and these combine to give:
.

.
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:
.

.
and that would give an answer of .

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