Question

7а-а²=1
а-?⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀

Answer 1

$7a - a {}^{2} = 1 \\ 7a - a - 1 = 0 \\ a {}^{2} - 7a + 1 = 0 \\ a = \frac{ - ( - 7) + - \sqrt{( - 7) {}^{2} - 4 \times 1 \times 1 } }{2 \times 1} \\ a1 = \frac{7 - 3 \sqrt{5} }{2} \\ a2 = \frac{7 - 3 \sqrt{5} }{2}$

Удачи!