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CBSE Class 12 Math 2013 Solved Paper

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Question : 11 of 29
Marks: +1, -0
Show that the function f in A = R - {23}\left\{ \frac{2}{3} \right\} defined as f(x) = 4x+36x4\frac{4x+3}{6x-4} is one-one and onto.
Hence find f1\overset{-1}{f}
Solution:  
f(x) = 4x+36x4\frac{4x+3}{6x-4}
Let f (x1)(x_1) = f (x2)(x_2)
4x1+36x14\frac{4x_1+3}{6x_1-4} = 4x2+36x24\frac{4x_2+3}{6x_2-4}
⇒ 24 x1x2x_1x_2 - 16 x1x_1 + 18 x2x_2 - 12 = 24 x1x2x_1x_2 + 18 x1x_1 - 16 x2x_2 - 12
⇒ 18 x2x_2 + 16 x2x_2 = 18 x1x_1 + 16 x1x_1
⇒ 34 x2x_2 = 34 x1x_1
Since, 4x1+36x14\frac{4x_1+3}{6x_1-4} is a real number, therefore, for every y in the co-domain of f, there exists a number x in R - {23}\left\{ \frac{2}{3} \right\} such that f(x) = y = 4x1+36x14\frac{4x_1+3}{6x_1-4}
Therefore, f(x) is onto.
Hence, f1f^{-1} exists.
Now, let y = 4x1+36x14\frac{4x_1+3}{6x_1-4}
⇒ 6xy - 4y = 4x + 3
⇒ 6xy - 4xx = 4y + 3
⇒ x (6y - 4) = 4y+ 3
⇒ x = 4y+36y4\frac{4y+3}{6y-4}
⇒ y = 4x+36x4\frac{4x+3}{6x-4} int exchanging the variables x and y
f1f^{-1} (x) = 4x+36x4\frac{4x+3}{6x-4} [put y = f1f^{-1} x]
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