ответ:
10 минут
объяснение:
последняя строка таблицы говорит о том что ванна полностью опорожнилась за 60 минут, т.е.
( \frac{1}{x+2} - \frac{1}{x})*60 = -1 \\
\frac{x-x-2}{x(x+2)}*60 = -1 \\
\frac{-2}{x(x+2)} = - \frac{1}{60} \\
x(x+2)=120 \\
x^{2} +2x-120=0 \\
d = 4 + 4*120 = 484 \\
\sqrt{d} = 22 \\
x_{1} = \frac{-2+22}{2}=10 \\
x_{2} = \frac{-2-22}{2}=-12 \\
второй корень посторонний.
ответ: второй кран опорожнит полную ванну за 10 минут.
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S13 = (a1+6d)*13 = 130a1+6d = 10 (2)Решим систему (1),(2).a1=70, d = -10