Mr and Mrs Davis have a son. His name is Bobby, and it was his birthday a few days ago.
Last Sunday Bobby's grandfather came to visit them, and he brought Bobby a nice toy train. When he gave it to Bobby, he said: "I'm sorry, Bobby, but I forgot your birthday last Tuesday, so I give you this present then."
"Oh, that doesn't matter Grandpa," Bobby answered. "Thank you very much. It's just what I wanted."
"And how old are you now, Bobby?" his grandfather asked. Bobby knew the answer to that question. "I'm five, Grandpa," he said.
"That's good," the old man said. "You're a big boy now, Bobby. And what are you going to be when you're older?" Bobby knew the answer to that question, too. "I' m going to be six, Grandpa," he answered
Этот?
x- cкорость течения
44/(20+x) + 54/(20-x) = 7-2
44(20-x) + 54(20+x)=5(400-x^2)
880 - 44x + 1080 + 54x = 2000 - 5x^2
1960 + 10x = 2000 - 5x^2
5x^2 + 10x - 40 = 0
x^2 + 2x - 8 = 0
x1 = 2 x2=-4
Ответ: 2 км/ч
А¹⁰·(а⁻³)⁴=а¹⁰а⁻¹²=а⁻²
(1/9)⁻²=9²=81
Ну так изи
x^2-1.44=0
(x-1.2)(x+1.2)=0
x-1.2=0 или x+1.2=0
x=1.2 x=-1.2