Let f : A -> B and g : B -> C be two functions. Then the composition of f and g denoted by g o f is defined as the function g o f (x) = g(f (x)) for all x ∈ A.
Generally, f o g ≠ g o f for any two functions f and g. So, composition of functions is not commutative.
Using the functions f and g given, find f o g and g o f. Check whether f o g = g o f .
Question 1 :
f(x) = x - 6 and g(x) = x2
Solution :
f o g = f[g(x)]
= f[x2]
= x2 - 6 ----(1)
g o f = g[f(x)]
= g[x- 6]
= (x - 6)2
= x2 - 2(x)(6) + 62
= x2 - 12x + 36 ----(2)
From (1) and (2), we see that f o g ≠ g o f.
Question 2 :
f(x) = 2/x and g(x) = 2x2 - 1
Solution :
f o g = f[g(x)]
= f[2x2 - 1]
= f(2x2 - 1)
= 2/(2x2 - 1) ----(1)
g o f = g[f(x)]
= g[2/x]
= 2(2/x)2 - 1
= 2(4/x2) - 1
= 8/x2 - 1 ----(2)
From (1) and (2), we see that f o g ≠ g o f.
Question 3 :
f(x) = (x + 6)/3 and g(x) = 3 - x
Solution :
f o g = f[g(x)]
= f[3 - x]
= (3 - x + 6)/3
= (9 - x)/3 ----(1)
g o f = g[f(x)]
= g[(x + 6)/3]
= 3 - [(x + 6)/3]
= 9/3 - [(x + 6)/3]
= [9 - (x + 6)]/3
= (9 - x - 6)/3
= (3 - x)/3 ----(2)
From (1) and (2), we see that f o g ≠ g o f.
Question 4 :
f(x) = 3 + x and g(x) = x - 4
Solution :
f o g = f[g(x)]
= f[x - 4]
= 3 + x - 4
= x - 1 ----(1)
g o f = g[f(x)]
= g[3 + x]
= 3 + x - 4
= x - 1 ----(2)
From (1) and (2), we see that f o g = g o f.
Question 5 :
f(x) = 4x2 - 1 and g(x) = 1 + x
Solution :
f o g = f[g(x)]
= f[1 + x]
= 4(1 + x)2 - 1
= 4(1 + 2x + x2) - 1
= 4 + 8x + 4x2 - 1
= 4x2 + 8x + 3 ----(1)
g o f = g[f(x)]
= g[4x2 - 1]
= 1 + 4x2 - 1
= 4x2 ---(2)
From (1) and (2), we see that f o g ≠ g o f.
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