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如图，在直角\(\Delta ABC\)中，点\(D\)为斜边\(BC\)的靠近点\(B\)的三等分点，点\(E\)为\(AD\)的中点，\(\left| {\overrightarrow {AB} } \right| = 3,\left| {\overrightarrow {AC} } \right| = 6\)．
求向量\(\overrightarrow {EB} \)与\(\overrightarrow {EC} \)夹角的余弦值．
参考答案：\( - \frac{{7\sqrt {130} }}{{130}}\)
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