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Date May 2018 Marks available 7 Reference code 18M.1.SL.TZ2.S_6
Level Standard Level Paper Paper 1 Time zone Time zone 2
Command term Find and Justify Question number S_6 Adapted from N/A

Question

Let f ( x ) = p x 2 + q x 4 p , where p ≠ 0. Find Find the number of roots for the equation f ( x ) = 0 .

Justify your answer.

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

METHOD 1

evidence of discriminant      (M1)
eg   b 2 4 a c , Δ

correct substitution into discriminant      (A1)
eg   q 2 4 p ( 4 p )

correct discriminant       A1
eg   q 2 + 16 p 2

16 p 2 > 0 ( accept p 2 > 0 )      A1

q 2 0 ( do not accept q 2 > 0 )      A1

q 2 + 16 p 2 > 0       A1

f has 2 roots     A1 N0

 

METHOD 2

y-intercept = −4p (seen anywhere)      A1

if p is positive, then the y-intercept will be negative      A1

an upward-opening parabola with a negative y-intercept      R1
eg  sketch that must indicate p > 0.

if p is negative, then the y-intercept will be positive      A1

a downward-opening parabola with a positive y-intercept      R1
eg  sketch that must indicate p > 0.

f has 2 roots     A2 N0

[7 marks]

 

Examiners report

[N/A]

Syllabus sections

Topic 2—Functions » SL 2.5—Modelling functions
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Topic 2—Functions

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