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f is a differentiable function for all x. Which of the following statements must be true?

a. d/dx ∫ f(x)dx=f(x)
b. d/dx ∫ f(t)dt= - f(x)
c. ∫ f′(x)dx=−f(x)

Answer :

The limits in the integral of the options are missing. They are :

a). [tex]\frac{d}{dx} \int^2_0 f(x)dx = 0[/tex]

b). [tex]\frac{d}{dx} \int^x_2 f(t)dt = 0[/tex]

c). [tex]$\int^x_2f'(x)dx=f(x)$[/tex]

Solution:

We known that

[tex]$\frac{d}{dx}\int^{g(x)}_{h(x)}f(t)dt = f(g(x))\cdot g'(x)-f(h(x))\cdot h'(x)$[/tex]

a).  [tex]\frac{d}{dx} \int^2_0 f(x)dx[/tex]

     [tex]$=f(2) \cdot \frac{d}{dx}(2) - f(0)\cdot \frac{d}{dx}(0)$[/tex]

     = 0 - 0

     = 0

Hence it is true.

b).   [tex]\frac{d}{dx} \int^x_2 f(t)dt[/tex]

      [tex]$=f(x)\frac{d}{dx}(x)-f(2)\cdot \frac{d}{dx}(2)$[/tex]

      [tex]$=f(x) \cdot 1 -f(2)\times 0 $[/tex]

      = f(x)

Hence it is true.

c). [tex]$\int^x_2f'(x)dx[/tex]

    [tex]$\left[f(x)\right]^x_2 = f(x)-f(2)$[/tex]

                [tex]$\neq f(x) $[/tex]

Hence it is false.

Therefore option a) and b). are true.