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$  x\overline{e}_{1}+y\overline{e}_{2}=x_{a}\overline{e}_{1}+y_{a}\overline{e}_{2}+t \cdot(x_{b}\overline{e}_{1}+y_{b}\overline{e}_{2}-(x_{a}\overline{e}_{1}+y_{a}\overline{e}_{2}))  $
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