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Yuri [45]
2 years ago
6

For the function f(x)=ax^3+bx^2-5x+9, determine the values of a and b so that f(-1)=12 and f'(-1)=3

Mathematics
1 answer:
Alecsey [184]2 years ago
4 0

If

<em>f(x)</em> = <em>ax</em> ³ + <em>bx</em> ² - 5<em>x</em> + 9

then

<em>f '(x)</em> = 3<em>ax </em>² + 2<em>bx</em> - 5

Given that <em>f</em> (-1) = 12 and <em>f</em> '(-1) = 3, we get the system of equations

-<em>a</em> + <em>b</em> + 5 + 9 = 12

3<em>a</em> - 2<em>b</em> - 5 = 3

or

-<em>a</em> + <em>b</em> = -2

3<em>a</em> - 2<em>b</em> = 8

Multiply through the first equation by 2 and add it to the second one to eliminate <em>b</em> and solve for <em>a</em> :

2(-<em>a</em> + <em>b</em>) + (3<em>a</em> - 2<em>b</em>) = 2(-2) + 8

-2<em>a</em> + 2<em>b</em> + 3<em>a</em> - 2<em>b</em> = -4 + 8

<em>a</em> = 4

Substitute this into the first equation above to solve for <em>b</em> :

-4 + <em>b</em> = -2

<em>b</em> = 2

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