FUNCTIONS 4

If F(x) = 8x + 10; Find F^-1(90).

Solution

HINT: F^-1(x) means inverse.

PROCEDURE:

Make x the subject and then interchange x and y variables.

Let y=F(x)

So,  y= 8x + 10

y – 10 = 8x

y – 10  = 8x

   8          8

y – 10   = x

   8

x  =   y – 10     after rearranging

           8

 y^-1 = x – 10     after interchanging x and y variables.

             8

F^-1(x) = x – 10     after interchanging x and y variables.

                8

Now we calculate F^-1(90) as hereunder;

F^-1(90) = 90 – 10     

                   8

F^-1(90) =  80      

                8

F^-1(90) =  10      [after dividing 80 by 8]

                

Hence, F^-1(90) =  10      

                              

TRY THIS……………………………

If F(x) = 7x + 20; Find F^-1(4).