Wzór:
[tex]S_n = \frac{2a_1 +r(n - 1)}{2} \times n[/tex]
Obliczenia:
[tex]S_{13} = \frac{2a_1 + 12 \times \frac{ \sqrt{3} }{2} }{2} \times 13 = 130 \sqrt{3} | \div 13 \\ a_1 + 3 \sqrt{3} = 10 \sqrt{3} \\ a_1 = 10 \sqrt{3} - 3 \sqrt{3} \\ a_1 = 7 \sqrt{3} [/tex]
