MathematicsDifferentiability of a FunctionJEE Advanced 1979Easy
Visualized Solution (English)
Visualizing the Piecewise Function
- Function: f(x)={2x2−7x+5x−1−31x=1x=1
- Goal: Find f′(1)
Factorizing the Denominator
- Denominator: 2x2−7x+5
- Splitting the middle term: 2x2−2x−5x+5
- Factoring: 2x(x−1)−5(x−1)=(x−1)(2x−5)
Simplifying f(x) for x=1
- For x=1: f(x)=(x−1)(2x−5)x−1
- Simplified form: f(x)=2x−51
The Derivative Definition
- Definition: f′(1)=limh→0hf(1+h)−f(1)
- Given: f(1)=−31
Calculating f(1+h)
- Substitute x=1+h into simplified f(x):
- f(1+h)=2(1+h)−51=2h−31
Setting up the Limit
- f′(1)=limh→0h2h−31−(−31)
- f′(1)=limh→0h2h−31+31
Simplifying the Numerator
- Numerator: 3(2h−3)3+(2h−3)=3(2h−3)2h
- Full expression: f′(1)=limh→03h(2h−3)2h
Cancelling the Indeterminate Form
- Cancelling h: f′(1)=limh→03(2h−3)2
Final Evaluation
- Substitute h=0: f′(1)=3(2(0)−3)2
- f′(1)=3(−3)2=−92
Summary and Key Takeaways
- Key Takeaway: Always use the limit definition for piecewise functions at junction points.
- Final Answer: f′(1)=−92
- Next Challenge: Check the differentiability if f(1) was changed to 0.
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