# Slope

• Jan 27th 2007, 07:20 AM
Mr_Green
Slope
If F(x) = 8^x contains the points A(a,3) and B(b,48), find the exact value of the slope of AB.

Could someone help me solve this please? It was on a practice SAT that I took, and I know I got it wrong cause I didn't know what to do.

Thanks
• Jan 27th 2007, 08:57 AM
earboth
Quote:

Originally Posted by Mr_Green
If F(x) = 8^x contains the points A(a,3) and B(b,48), find the exact value of the slope of AB.

Could someone help me solve this please? It was on a practice SAT that I took, and I know I got it wrong cause I didn't know what to do.

Thanks

Hello,

the coordinates of the points have to satisfy the equation of the function:

to A: $\displaystyle 3=8^a \Longrightarrow a=\log_{8}{3}$

to B: $\displaystyle 48=8^b \Longrightarrow b=\log_{8}{48}$

The slope is calculated by: $\displaystyle m=\frac{y_B-y_A}{b-a}$ Therefore:

$\displaystyle m=\frac{48-3}{\log_{8}{48}-\log_{8}{3}}=\frac{45}{\log_{8}{\left( \frac{48}{3} \right)}}$=$\displaystyle \frac{45}{\frac{\ln{\left( \frac{48}{3} \right)}}{\ln(8)}}=\frac{45 \cdot \ln(8)}{\ln(16)}=\frac{45 \cdot 3\ln(2)}{4\ln(2)}=\frac{135}{4}$

EB