To simplify the expression \(\left(\frac{y^{5}}{y^{3}}\right)^{3}\cdot\left(\frac{y^{7}}{y^{4}}\right)^{5}\), we can follow these steps:
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Simplify each fraction:
- \(\frac{y^{5}}{y^{3}} = y^{5-3} = y^{2}\)
- \(\frac{y^{7}}{y^{4}} = y^{7-4} = y^{3}\)
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Substitute back into the original expression: \[ \left(y^{2}\right)^{3} \cdot \left(y^{3}\right)^{5} \]
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Apply the power of a power rule:
- \(\left(y^{2}\right)^{3} = y^{2 \cdot 3} = y^{6}\)
- \(\left(y^{3}\right)^{5} = y^{3 \cdot 5} = y^{15}\)
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Multiply the results: \[ y^{6} \cdot y^{15} = y^{6 + 15} = y^{21} \]
Thus, the fully simplified expression is:
\[ \boxed{y^{21}} \]