Brooke_army
If y=x3+5x and dx/dt=5, find dy/dt when x=2.



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Brooke_army
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ok for this question i think im doing something wrong
i got the answer of 17 but this is not right
please help

nubeer
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can you find dy/dx of the given equation?

Brooke_army
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dy/dt=3x^2+5

phi
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when you take the derivative wrt to t
you should get a dx/dt term

hartnn
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u need to use chain rule
d/dt(5x) = 5 d/dx (dx/dt)

phi
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instead of d/dx x^3 = 3 x^2 dx/dx you do
d/dt x^3 = 3 x^2 dx/dt

phi
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if you think
\[ \frac{d}{dx} x= 1\]
think this
\[ \frac{ d}{dx} x= \frac{dx}{dx}= 1\]

sunnymony
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dw:1352559750685:dw

phi
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using the same logic
\[ \frac{d }{dt} x= \frac{dx}{dt} \]

sunnymony
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3x2+5 is ans

Brooke_army
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thanks so much i got the wright answer!!

sunnymony
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dy/dx= 3x2+5
dx/dt = 5
so both multiply we get =15x2+25

phi
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then you got
dy/dt=3x^2 dx/dt + 5 dx/dt
replace x=2, dx/dt=5 to get
dy/dt = 3*4*5+ 5*5= 85

sunnymony
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so 15( 2x2)+25
=60+25
85

sunnymony
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thats right
@phi