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Sprendimas:

$$V = S_{PAGR}\cdot H$$

$$S_{PAGR} = \frac{V}{H} = \frac{28\cdot \sqrt {3}}{7}$$

Lygiakraščio trikampio plotas $$S = \frac{a^{2}\cdot \sqrt {3}}{4}$$

$$\frac{a^{2}\cdot \sqrt {3}}{4} = \frac{28\cdot \sqrt {3}}{7}$$

 $$\frac{a^{2}}{4} = \frac{28}{7}$$

$$a^{2} = \frac{28\cdot 4}{7}$$

$$a^{2} = 4\cdot 4$$

$$a^{2} = 16$$

$$a = 4$$

Atsakymas: 4