Derivative of a Product

Calculus Level 2

If f ( x ) = ( x + 2 ) x 2 f(x) = (x+2) \sqrt{x-2} , then what is the value of f ( 11 ) ? f'(11)?

29 2 6 \frac{29\sqrt{2}}{6} 29 6 \frac{29}{6} 31 6 \frac{31}{6} 31 2 6 \frac{31\sqrt{2}}{6}

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4 solutions

F ( x ) = ( x + 2 ) x 2 F ( x ) = ( x + 2 ) ( x 2 ) 1 2 N o w w e t a k e t h e d e r i v a t i v e f ( x ) = ( x + 2 ) a n d g ( x ) = ( x 2 ) 1 2 F ( x ) = f ( x ) g ( x ) + f ( x ) g ( x ) F ( x ) = 1 ( x 2 ) 1 2 + ( x + 2 ) ( 1 2 ( x 2 ) 1 2 ) 1 F ( x ) = x 2 + x + 2 2 x 2 N o w w e s u b s t i t i t e F ( 11 ) = 11 2 + 11 + 2 2 11 2 F ( 11 ) = 9 + 13 2 9 F ( 11 ) = 3 + 13 6 F ( 11 ) = 18 + 13 6 F ( 11 ) = 36 6 F(x)=(x+2)*\sqrt { x-2 } \\ F(x)=(x+2)*{ (x-2) }^{ \frac { 1 }{ 2 } }\\ \\ Now\quad we\quad take\quad the\quad derivative\\ f(x)=(x+2)\quad and\quad g(x)={ (x-2) }^{ \frac { 1 }{ 2 } }\\ \\ F'(x)=f'(x)*g(x)+f(x)*g'(x)\\ F'(x)=1*{ (x-2) }^{ \frac { 1 }{ 2 } }+(x+2)*(\frac { 1 }{ 2 } { (x-2) }^{ -\frac { 1 }{ 2 } })*\quad 1\\ F'(x)=\sqrt { x-2 } +\frac { x+2 }{ 2\sqrt { x-2 } } \\ Now\quad we\quad substitite\\ F'(11)=\sqrt { 11-2 } +\frac { 11+2 }{ 2\sqrt { 11-2 } } \\ F'(11)\quad =\quad \sqrt { 9 } +\quad \frac { 13 }{ 2\sqrt { 9 } } \\ F'(11)\quad =\quad 3+\frac { 13 }{ 6 } \\ F'(11)\quad =\quad \frac { 18+13 }{ 6 } \\ F'(11)\quad =\quad \frac { 36 }{ 6 }

Nishant Rawat
Feb 6, 2014

f(x)= (x+2)*(x-2)^.5 --------eq. 1

Let (x-2)^.5=y That makes x-2=y^2 which makes x+2=y^2+4

Taking the derivative of this relation between gives us dx=2y*dy Hence, dy/dx= 1/2y

The derivative of y is 0.5/(x-2)^.5

Substituting y in equation 1 gives you f(x)= (y*(y^2+4) = y^3+4y

Now finding the derivative of f(x) gives us f'(x) =(3y^2 + 4) (dy/dx) Substituting x back gives f'(x)=(3(x-2)+4) (1/(2(x-2)^.5)) Putting x=11 we get 31/6

how ? x+2=y^2+4

Imran khan - 7 years, 1 month ago

What?

Y'(x) = f(x).g '(x) + f '(x).g(x)

(x-2).1/2(x-2)^-1/2 .1 + 1.(x-2)^1/2

Y '(11) = 9/2 (11-2)^1/2 + 9^1/2 = 9+ 18/6 = 27/6

my calculations are wrong ?

Tibrôncio Cavalcante - 6 years, 5 months ago
Karan Joisher
Jul 11, 2014

If y = uv then apply UV rule --> u * dv/dx + v * du/dx

Nisarg Shah
Feb 10, 2014

f'(x)=3x-2/(2*((x-2)^1/2)

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