Dabour Hub EGEgyptian Curriculum 2026/2027
2-2 (e) Finding unknown constants from extrema

2-2 (e) Finding unknown constants from extrema

Point 7 of 7 — Increasing, Decreasing Functions and Maximum and Minimum ValuesQuestion bank: 0 questions (0 with pictures)

07 2-2 (e) Finding unknown constants from extrema

Idea

  • Use the facts "the point lies on the curve" and "f′(c)=0 at the extremum" to build equations.
✍️ Worked examples

Example 15: The curve f(x)=ax3+bx2 has a local maximum at (1,4). Find a and b.

Solution:

  • The point lies on the curve: a+b=4.
  • Critical point: f′(x)=3ax2+2bx, so f′(1)=3a+2b=0.
  • From the first equation b=4−a; then 3a+8−2a=0, so a=−8 and b=12.
  • Check: f′(x)=−24x2+24x=24x(1−x) is positive just before x=1 and negative just after, so x=1 is indeed a maximum.

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