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21M.1.SL.TZ1.5

pestleMathematicsAASLPaper 121M· sl-5-4-tangents-and-normalsource ↗

Consider the functions fx=-x-h2+2k and gx=ex-2+k where h,k.

The graphs of f and g have a common tangent at x=3.

Find f'x.

[1]
a.

Show that h=e+62.

[3]
b.

Hence, show that k=e+e24.

[3]
c.
Markscheme / solution

f'x=-2x-h          A1

 

[1 mark]

a.

g'x=ex-2  OR  g'3=e3-2 (may be seen anywhere)          A1

 

Note: The derivative of g must be explicitly seen, either in terms of x or 3.

 

recognizing f'3=g'3          (M1)

-23-h=e3-2  =e

-6+2h=e  OR  3-h=-e2          A1

 

Note: The final A1 is dependent on one of the previous marks being awarded.

 

h=e+62          AG

 

[3 marks]

b.

f3=g3          (M1)

-3-h2+2k=e3-2+k

correct equation in k

 

EITHER

-3-e+622+2k=e3-2+k          A1

k=e+6-e-622 =e+-e22          A1

 

OR

k=e+3-e+622          A1

k=e+9-3e-18+e2+12e+364          A1

 

THEN

k=e+e24          AG

 

[3 marks]

c.
Examiners’ report
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b.
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c.