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NCERT Class XI Mathematics - Relations and Functions - Solutions

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Question : 25 of 36
Marks: +1, -0
The relation f is defined by f (x) = {x2,0x33x,3x10\begin{cases} x^2, & 0 \le x \le 3 \\ 3x, & 3 \le x \le 10 \end{cases}
The relation g is defined by g (x) = {x2,0x23x,2x10\begin{cases} x^2, & 0 \le x \le 2 \\ 3x, & 2 \le x \le 10 \end{cases}
Show that f is a function and g is not a function.
Solution:  
(i) Given relation is
f (x) = {x2,0x33x,3x10\begin{cases} x^2, & 0 \le x \le 3 \\ 3x, & 3 \le x \le 10 \end{cases}
Here f (x) = x2x^2 for all x ∈ [0, 3] ⇒ f(3) = 323^2 = 9
and f (x) = 3x for all x ∈ [3, 10] ⇒ f(3) = 3 × 3 = 9.
It shows that f (x) takes unique value at each point in its domain [0, 10].
Hence, f (x) is a function.
(ii) Given relation is
g (x) = {x2,0x23x,2x10\begin{cases} x^2, & 0 \le x \le 2 \\ 3x, & 2 \le x \le 10 \end{cases}
Here, g(x) = x2x^2 ∀ x ∈ [0, 2]
⇒ g(2) = 222^2 = 4 and g(x) = 3x ∀ x ∈ [2, 10] ⇒ g(2) = 3 × 2 = 6
It shows that g(x) does not take unique value at point 2. Hence, g(x) is not a function.
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