6
$\begingroup$

I have the following function $$f(x) = 3x^3 - 5x^2 - 4x + 4.$$

I would like to find the value of $f(x)$ when $x = -3$.

I have ordered the equation in the following way. $$f(-3) = (3 \times -3^3) - (5 \times -3^2) - (4 \times -3) + 4 = -28.$$

Can anyone tell me if the following syntax is correct and if not, where have I gone wrong.

Many Thanks.

$\endgroup$
  • $\begingroup$ Is your function supposed to be $f(x) = 3x^3 -5x^2-4x+4$? $\endgroup$ – Arturo Magidin Feb 23 '11 at 20:53
  • $\begingroup$ yes correct thats exactly how it appears on my paper. $\endgroup$ – cocacola09 Feb 23 '11 at 20:54
  • $\begingroup$ You wrote $f(x)=3x^2-...$ (squared, not cubed). That's why I asked. $\endgroup$ – Arturo Magidin Feb 23 '11 at 20:56
4
$\begingroup$

When you write $5*-3^2$, the precedence of operations means that you first compute $3^2$, then you multiply the result by $-1$, then you multiply the entire thing by $5$. This is not what you want.

Don't be scared of parentheses! The best way to write this, if you want to insist on putting the $*$ in it (we normally just write multiplication by juxtaposition) would be: $$f(-3) = (3*(-3)^3) - (5*(-3)^2) - (4*(-3)) + 4.$$ More succinctly, you can write it as: $$f(-3) = 3(-3)^3 - 5(-3)^2 - 4(-3) + 4.$$ By the precedence order, you would first compute the exponentials, then the products, then the sums.

That said, I don't think you computed $f(-3)$ correctly.

$\endgroup$
  • $\begingroup$ Hey thank you again for the reply, I remember you helped me before. $\endgroup$ – cocacola09 Feb 23 '11 at 22:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.