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Using the definition of the derivative, find f(x)f'(x). \newlinef(x)=x2+6x7f(x)=-x^{2}+6x-7

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Q. Using the definition of the derivative, find f(x)f'(x). \newlinef(x)=x2+6x7f(x)=-x^{2}+6x-7
  1. Definition of Derivative: The definition of the derivative f(x)f'(x) is the limit as hh approaches 00 of the difference quotient f(x+h)f(x)h\frac{f(x+h) - f(x)}{h}. Let's apply this definition to the function f(x)=x2+6x7f(x) = -x^2 + 6x - 7.
  2. Calculate f(x+h)f(x+h): First, we need to calculate f(x+h)f(x+h). This means we substitute x+hx+h into the function in place of xx:f(x+h)=(x+h)2+6(x+h)7.f(x+h) = -(x+h)^2 + 6(x+h) - 7.
  3. Expand f(x+h)f(x+h): Now, let's expand f(x+h)f(x+h):f(x+h)=[(x+h)(x+h)]+6(x+h)7f(x+h) = -[(x+h)(x+h)] + 6(x+h) - 7=(x2+2xh+h2)+6x+6h7.= -(x^2 + 2xh + h^2) + 6x + 6h - 7.
  4. Form Difference Quotient: Next, we form the difference quotient [f(x+h)f(x)]/h[f(x+h) - f(x)]/h:[f(x+h)f(x)]h=[x22xhh2+6x+6h7][x2+6x7]h\frac{[f(x+h) - f(x)]}{h} = \frac{[-x^2 - 2xh - h^2 + 6x + 6h - 7] - [-x^2 + 6x - 7]}{h}.
  5. Simplify Difference Quotient: We simplify the difference quotient by canceling out the terms that appear in both f(x+h)f(x+h) and f(x)f(x):f(x+h)f(x)h=x22xhh2+6x+6h7+x26x+7h\frac{f(x+h) - f(x)}{h} = \frac{-x^2 - 2xh - h^2 + 6x + 6h - 7 + x^2 - 6x + 7}{h}\= \frac{2-2xh - h^22 + 66h}{h}.
  6. Factor Out hh: Now, we can factor out hh from the numerator: f(x+h)f(x)h=h(2xh+6)h\frac{f(x+h) - f(x)}{h} = \frac{h(-2x - h + 6)}{h}.
  7. Take Limit: We cancel hh from the numerator and denominator: f(x+h)f(x)h=2xh+6\frac{f(x+h) - f(x)}{h} = -2x - h + 6.
  8. Take Limit: We cancel hh from the numerator and denominator: f(x+h)f(x)h=2xh+6\frac{f(x+h) - f(x)}{h} = -2x - h + 6. Finally, we take the limit as hh approaches 00: f(x)=limh0[2xh+6]f'(x) = \lim_{h\to0} [-2x - h + 6] = 2x+6-2x + 6.

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