SOLUTION: if a graph of f(x)=-3x2+2x+k passes through the point (-1,3), then the value of k is?

Question 92417: if a graph of f(x)=-3x2+2x+k passes through the point (-1,3), then the value of k is?
Answer by bucky(2189) (Show Source): You can put this solution on YOUR website!
Given:
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Substitute y for f(x) to convert the given equation to the equivalent form:
.

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You are told that the point (-1,3) is on the graph. That means that when x is -1, the corresponding
value of y is 3. So you can substitute -1 for x and 3 for y and the equation should still
be true. Make these two substitutions and the equation becomes:
.

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Square the minus one on the right side to get -1*-1 = +1 and the equation becomes:
.

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Multiply the first two terms on the right side and you get:
.

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Combine the first two terms on the right side and the result is:
.

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Get rid of the -5 on the right side by adding +5 to cancel it out. But if you add +5
to the right side, you must also add +5 to the left side. Adding +5 to both sides gives
you:
.

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and that's your answer.
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You can check this by replacing k by +8 in the original equation to get:
.

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Then let x = -1 and see if the left side is +3 as the problem said that y should be +3
when x is -1.
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Hope this helps you to see your way through this problem.
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