SOLUTION: Evaluate 3n^2-10n-9, given n(n-3)=28

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Question 1177229: Evaluate 3n^2-10n-9, given n(n-3)=28
Found 2 solutions by MathLover1, Theo:
Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!

Evaluate 3n%5E2-10n-9, given n%28n-3%29=28
first
n%28n-3%29=28+.........solve for n
n%5E2-3n=28
n%5E2-3n-28=0
%28n+-+7%29+%28n+%2B+4%29+=+0
solutions: n=7 or n=-4
Evaluate
3n%5E2-10n-9......if n=7
3%2A7%5E2-10%2A7-9=68

3n%5E2-10n-9......if n=-4
3%2A%28-4%29%5E2-10%2A%28-4%29-9=79


Answer by Theo(13342) About Me  (Show Source):
You can put this solution on YOUR website!
first you need to solve for n in the equation of n * (n-3) = 28
simplify this to n^2 - 3n = 28
subtract 28 from both sides of the equation to get:
n^2 - 3n - 28 = 0
solve for n in this quadratic equation to get:
n = -4 or n = 7.

when n = -4, 3n^2 - 10n - 9 = 79.
when n = 7, 3n^2 - 10n - 9 = 68.

those should be your answers.

i solved this graphically using the desmos.com calculator.

this is what the results of that analysis look like.



in the graph, i used x instead of n.