# Math

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An airplane has a speed of 400 km/h with no wind. The airplane flies 2140 km with the wind. The airplane can only fly 1860 km against the wind in the same time. If w equals the speed of the wind, which equation would be used to find w?

a) 2140/(w+400) = 1860/(w-400)
b. 2140/(400+w) - 1860/(w-400)=2140-1860/w

c) 2140/(400+w) = 1860/(400-w)
d) 2140/(400+w) - 1860/(400-w) = 650

2. What is the average rate of change of f(x)= (x-5)/(x+3) from x=-2 to x =4?

b) 8/7
C) -25/21
D) -11/3

Can someone explain to me what to do with the denominators? that's the only thing I am confused about. I know that you plug in -2 for x, f(x)= (x-5)/(x+3)

Y2-Y1/X2-X1

• Math -

anyone?? D:

• Math -

I assume you got C for #1.
Time = distance/speed
The two times are equal

#2.
f(-2) = -7/1
f(4) = -1/7

So, over an interval of 6 units, y changed from -7 to -1/7 = +48/7

So, the average change per unit is (48/7)/6 = 8/7

• Math -

How did you get the denominator 1 and 7? That is what I am confused :3

• Math -

You're kidding, right?

f(x)= (x-5)/(x+3)

f(-2) = (-2-5)/(-2+3) = -7/1
f(4) = (4-5)/(4+3) = -1/7

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