What is the right/pythonic way to do math on a few indexed elements in a Pandas Dataframe?

I tried a few ways but they seem awkward and confusing:

df = pd.DataFrame({'x': [1, 2, 3, 4, 5, 6, 7, 9, ]})
df['y']=  df['x']/2

print(df)
def printEpr(x):
    print(F"""End Point Rate: 
    {epr} dy/dx""")    

epr =(df['y'][7] - df['y'][0])/(df['x'][7] - df['x'][0]) 
printEpr(epr)

epr = (df.iloc[[-1],[1]].values  - df.iloc[[0],[1]].values)  / (df.iloc[[-1],[0]].values - df.iloc[[1],[0]].values)
printEpr(epr)

#epr = (df['y'] - df.iloc[0,'y'])/(df.iloc[-1,'x'] - df.iloc[0,'x'])
#printEpr(epr)

yind = df.columns.get_loc('y')
xind = df.columns.get_loc('x')
epr = (df.iloc[-1,yind] - df.iloc[0,yind])/(df.iloc[-1,xind] - df.iloc[0,xind])
printEpr(epr)

#epr = (df.iloc[-1,'y'] - df.iloc[0,'y'])/(df.iloc[-1,'x'] - df.iloc[0,'x'])
#printEpr(epr)

With results:

   x    y
0  1  0.5
1  2  1.0
2  3  1.5
3  4  2.0
4  5  2.5
5  6  3.0
6  7  3.5
7  9  4.5
End Point Rate: 
    0.5 dy/dx
End Point Rate: 
    [[0.57142857]] dy/dx
End Point Rate: 
    0.5 dy/dx