The approximate value of the function $f(x)=x^{3}-3 x+5$ at $x=1.99$ is

The approximate value of the function $f(x)=x^{3}-3 x+5$ at $x=1.99$ is
  1. 6.91
  2. 6.94
  3. 7.94
  4. 7.91

Solution

$f(x)=x^{3}-3 x+5 \Rightarrow f^{\prime}(x)=3 x^{2}-3$ Let $a=2, h=-0.01$ $\therefore f(a)=f(2)=2^{3}-3(2)+5=7$ $f^{\prime}(a)=f^{\prime}(2)=3(2)^{2}-3=9$ We know that $f(a+h) = f(a)+h f^{\prime}(a)$ $f(1.99) = 7-(0.01)(9) = 7-0.09 = 6.91$

Asked in: MHT CET 2020 (16 Oct Shift 1)

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