Concave mirrors - where is the object

[danago]

"A concave mirror of focal length 1m is used to magnify an object by a factor of 4. Where is the object in relation to the pole of the mirror?"

Heres what i did:
<br /> \begin{array}{c}<br /> M = \frac{v}{u} \\ <br /> \therefore 4 = \frac{v}{u} \\ <br /> \therefore v = 4u \\ <br /> \end{array}<br />

<br /> \begin{array}{c}<br /> \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \\ <br /> \therefore 1 = \frac{1}{u} + \frac{1}{{4u}} = \frac{5}{{4u}} \\ <br /> \therefore u = 1.25m \\ <br /> \end{array}<br />

Where am i going wrong? According to my textbook, the answer is 0.75m.

Thanks in advance,
Dan.

 

[Alt+F4]

You are forgeting that M = -di/do

Formula = (1/do + 1/di) = (1/f)

So 4do = - di

1/do - 1/4do = 1

1/4do - 4/4do = 1

3/4do = 1

do = .75

 

[TheloniousMONK]

3/4do = 1

do = .75

Right.

3/(4do) = 1 so do = .75