# Thread: Can't figure out these problems

1. ## Can't figure out these problems

I've got these two problems which I can't seem to figure out. I've tried different ways of doing them but none give me the correct answer.

Here is the first problem

f(x) = 6.0 - 8.0x2; find f(9.0 x 10.0) - [9.0f(10.0)]2

And the second

A conical sheet metal hood is to cover an area 1.0 m in diameter. What is the surface area of the hood if its height is 8.0?

There are a number of questions similar to the first, so I'm hoping once I figure out how to do this one the others will make more sense.

Thanks for any help!

2. ## Re: Can't figure out these problems

If $f(x)=6-8x^2$
then $f(90)=6-8\times90^2=-64794$
and $f(10)=6-8\times10^2=-794$

So with that information
$f(9.0 \times 10.0) - (9.0f(10.0))^2=f(90)-(9f(10))^2$
$f(90)-(9f(10))^2=-64794-(9\times(-794))^2=-64794-51065316 =-51130110$

In the second question the curved surface area of a cone is equal to $\pi r\sqrt{r^2+h^2}$ where r is the radius of the base and h is the height.

4. ## Re: Can't figure out these problems

ah ty for the help, Im able to do all the questions similar to the first now, and also feel pretty dumb about the second... didnt even clue in that conical meant cone so I was trying to figure out the area for other shapes and seeing if they matched any of the answers lol