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# Find the requested composition or operation.

f(x) = 5-x, g(x) = -9x+7

Find (f + g)(x).

### 4 Answers by Expert Tutors

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Shelly J. | Excellent Maths Tutoring for academic successExcellent Maths Tutoring for academic su...
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Hi Jay,

f(x)=5-x
g(x)=-9x+7

(f+g)(x)=f(x)+g(x)

=5-x-9x+7

=12-10x
Philip P. | Effective and Affordable Math TutorEffective and Affordable Math Tutor
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If you meant the sum of f(x) + g(x), then the answers below are correct.  If you meant the composition of f(x) and g(x), that is f(g(x)) or f ° g, then substitute g(x) for x in f(x):

f ° g = 5 - (-9x + 7) = 9x - 2

Note that composition does not obey the associative law, that is f ° g does not equal g ° f
Ebenezer O. | Aerospace Engr & Air Traffic Control Grad For General Ed. TutoringAerospace Engr & Air Traffic Control Gra...
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Given
f(x) = 5 - x
g(x) = -9x + 7

(f + g)(x)

f(x) + g(x) = (5 - x) + (-9x + 7)

f(x) + g(x)  = (5 - x - 9x +7)

f(x) + g(x)  = (12 - 10x)

this can be rewritten as;

(f + g)(x) = (12 - 10x)
David T. | Experienced Math and Physics TutorExperienced Math and Physics Tutor
4.8 4.8 (12 lesson ratings) (12)
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(f+g)(x) is the same as f(x) + g(x).

It is just slightly shorter notation, and removes any confusion as to whether f and g use the same variable when evaluating.