Originally Posted by

**sammy28** hi all,

im confused with the way an answer is given in reciprocal cm format the questions was:

The temperature $\displaystyle \theta C$ measured at a distance $\displaystyle x cm$ is given by:

$\displaystyle \theta=16+\frac{450}{x^\frac{3}{2}}$

for distances $\displaystyle x\geq 1$

Find the temeparture gradient (i.e. the rate at which temperature decreases with distance) 10cm from the flame.

my answer was

$\displaystyle t=f(d)=16+450x^{-\frac{3}{2}}$, where t(emp), d(istance)

$\displaystyle f'(d)=-675x^{-\frac{5}{2}}$

$\displaystyle f'(10)=-675.\frac{1}{100\sqrt{x}}$

$\displaystyle =-\frac{6.75}{\sqrt{10}} \equiv -0.675\sqrt{10}$

$\displaystyle =-2.13C/cm$ to 3 sf

but the book (Edexcel A level maths} gives the answer as

$\displaystyle =-2.13C/cm^{-1}$

can someone please explain what the reciprocal cm is all about.

thanks.