3. Решите уравнение: a) x² - 20² = 0 б) 81 – 16y² = 0 в) (5-y)²-y(y+8,1)=25

Решение:

1. \( x^2 - 20^2 = 0 \)
   \( x^2 - 400 = 0 \)
   \( x^2 = 400 \)
   \( x = \pm \sqrt{400} \)
   \( x = \pm 20 \)
2. \( 81 - 16y^2 = 0 \)
   \( 16y^2 = 81 \)
   \( y^2 = \frac{81}{16} \)
   \( y = \pm \sqrt{\frac{81}{16}} \)
   \( y = \pm \frac{9}{4} \)
3. \( (5-y)^2 - y(y+8.1) = 25 \)
   \( 25 - 10y + y^2 - y^2 - 8.1y = 25 \)
   \( 25 - 10y - 8.1y = 25 \)
   \( -10y - 8.1y = 25 - 25 \)
   \( -18.1y = 0 \)
   \( y = 0 \)

Ответ: a) x = ±20; б) y = ±9/4; в) y = 0.