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8.5: Стабільність у напрузі
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\[\delta V = \delta (lA) = \delta lA + l \delta A = 0 \] \[\delta \epsilon = \frac{\delta l}{l}= − \frac{\delta A}{A} \] \[\delta \prod = \int_{V} \sigma \delta \epsilon dv + Pl \frac{\delta A}{A} \] ...\[\delta V = \delta (lA) = \delta lA + l \delta A = 0 \] \[\delta \epsilon = \frac{\delta l}{l}= − \frac{\delta A}{A} \] \[\delta \prod = \int_{V} \sigma \delta \epsilon dv + Pl \frac{\delta A}{A} \] \[\delta^2 \prod = \int_{V} \delta \sigma \delta \epsilon dv − Pl \frac{\delta A \delta A}{A^2} \] \[lA \delta \sigma \delta \epsilon = Pl \delta \epsilon \delta \epsilon \] \[\delta \sigma = \frac{P}{A} \delta \epsilon = \sigma \delta \epsilon \]