Всем привет! Народ помогите решить две задачи.
#1. A barge moves 7 km/h in still water. It travels 45 km. upriver and 45 km. downriver in a total time of 14 hr. What is the speed of the current?
#2. A freight train covered 120 mi at a certain speed. Had the train been able to travel 10 mph faster, the trip would have been 2 hr shorter. How fast did the train go?

2 km/h

#1 2 km/h

#1 - 2 km/h
#2 - 20 mi/h

напишите пожалуйста как решыть? ход?

#1
Time taken = 45/(7+x) + 45/(7-x) = 14
Multiply through by (7+x)(7-x):
45(7-x) + 45(7+x) = 14(7+x)(7-x)
Expand the brackets and collect terms:
630 = 14(49-x²
Divide by 14:
45 = 49-x²
x²=4
x=2

The speed of the current is 2 km/h.

Check: time taken = 45/5 + 45/9 = 9+5 = 14 hr.
#2.
D=RT
D/R=T
120/X=T
120/(X+10)=T-2
NOW REPLACE T WITH 120/X IN THE SECOND EQUATION & SOLVE FOR X.
120/(X+10)=(120/X)-2
120/(X+10)=(120-2X)/X NOW CROSS MULTIPLY.
(X+10)(120-2X)=120X
120X+1200-20X-2X^2=120X
-2X^2+100X+1200-120X=0
-2X^2-20X+1200=0
-2(X^2+10X-600)=0
-2(X-20)(X+30)=0
X-20=0
X=20 MPH. ANS. FOR THE FIRST 120 MILE TRIP.
20+10=30 MPH. ANS. FOR THE FASTER TRIP.
PROOF:
120/30=(120/20)-2
4=6-2
4=4

7 класс?

школа вспомнилась

Огромное всем СПАСИБО!!!!

Димон
следующий раз ты сразу в гугл вбивай свои вопросы
не думаю что кто то тут сидел решал
))0

nuzhno pomoch' cheloveku, dazhe esli prosto skopirovat' s google