Suppose $\|u\|=1=\|3v\|$ . If $\|u+v\|=1$ then $u \cdot v=?$

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For $\|3v\| = 1$, I isolated $v$ and got: $\|v\| = 1/3$.

I used $\|u+v\|^2 = (u+v)\cdot(u+v)$ and got to

$$\|u+v\|^2 = \|u\|^2 + 2(u \cdot v) + \|v\|^2$$

$$(1) = (1) + 2(u \cdot v) + (1/9)$$

$$u \cdot v = -1/18$$

Is my answer/process correct?