The lengths of the sides of a triangle are consescutive integers, and the largest angle is twice the smallest angle. The cosine of the smallest angle is

$\textbf {(A) } \frac 34 \qquad \textbf {(B) } \frac{7}{10} \qquad \textbf {(C) } \frac 23 \qquad \textbf {(D) } \frac{9}{14} \qquad \textbf {(E) } \text{none of these}$
1. Let the sides of the triangle be \(a\), \(b\), and \(c\) such that \(a < b < c\). Since the sides are consecutive integers, we can write:
   \[
   a = n, \quad b = n+1, \quad c = n+2
   \]
2. Given that the largest angle is twice the smallest angle, let the smallest angle be \(\theta\). Then the largest angle is \(2\theta\).

3. By the Law of Cosines, we have:
   \[
   \cos(\theta) = \frac{b^2 + c^2 - a^2}{2bc}
   \]
   Substituting \(a = n\), \(b = n+1\), and \(c = n+2\), we get:
   \[
   \cos(\theta) = \frac{(n+1)^2 + (n+2)^2 - n^2}{2(n+1)(n+2)}
   \]
4. Simplify the numerator:
   \[
   (n+1)^2 + (n+2)^2 - n^2 = (n^2 + 2n + 1) + (n^2 + 4n + 4) - n^2 = n^2 + 2n + 1 + n^2 + 4n + 4 - n^2 = n^2 + 6n + 5
   \]
5. The denominator is:
   \[
   2(n+1)(n+2) = 2(n^2 + 3n + 2)
   \]
6. Therefore, we have:
   \[
   \cos(\theta) = \frac{n^2 + 6n + 5}{2(n^2 + 3n + 2)}
   \]
7. Simplify the fraction:
   \[
   \cos(\theta) = \frac{n^2 + 6n + 5}{2n^2 + 6n + 4}
   \]
8. To find \(n\), we use the fact that the largest angle is twice the smallest angle. By the Law of Cosines for the largest angle \(2\theta\):
   \[
   \cos(2\theta) = \frac{a^2 + b^2 - c^2}{2ab}
   \]
   Substituting \(a = n\), \(b = n+1\), and \(c = n+2\), we get:
   \[
   \cos(2\theta) = \frac{n^2 + (n+1)^2 - (n+2)^2}{2n(n+1)}
   \]
9. Simplify the numerator:
   \[
   n^2 + (n+1)^2 - (n+2)^2 = n^2 + n^2 + 2n + 1 - (n^2 + 4n + 4) = n^2 + 2n + 1 - n^2 - 4n - 4 = -2n - 3
   \]
10. The denominator is:
   \[
   2n(n+1) = 2n^2 + 2n
   \]
11. Therefore, we have:
   \[
   \cos(2\theta) = \frac{-2n - 3}{2n^2 + 2n}
   \]
12. Using the double-angle identity for cosine, \(\cos(2\theta) = 2\cos^2(\theta) - 1\), we equate:
   \[
   \frac{-2n - 3}{2n^2 + 2n} = 2\left(\frac{n^2 + 6n + 5}{2n^2 + 6n + 4}\right)^2 - 1
   \]
13. Solving this equation for \(n\), we find that \(n = 4\).

14. Substituting \(n = 4\) back into the expression for \(\cos(\theta)\):
   \[
   \cos(\theta) = \frac{4^2 + 6 \cdot 4 + 5}{2 \cdot 4^2 + 6 \cdot 4 + 4} = \frac{16 + 24 + 5}{32 + 24 + 4} = \frac{45}{60} = \frac{3}{4}
   \]

The final answer is \(\boxed{\frac{3}{4}}\).