If we have the result as the title, then I can solve my real question. The original question was stated as follows.

In a paper I found the following lemma：

Let $S$ be a nonsingular projective surface, $R\in PicS$ a divisor with $R^2>0$ . Let

$(E_{i})$ be the family of distinct curves such that $R\cdot E_{i}=0$.

Then the $E_{i}$ are numerically independent.

The proof just says that the result follows from Hodge Index Theorem. But I cant see how. HIT just assert that the intersection matrix $(E_i.E_j)$ is negative semidefinite. Why $E_i$ can't be numerically dependent?

Any hint is welcome. Thanks a lot.