Question

why magnification is taken as negative for real image and taken as positive for virtual image

Answer 1

Magnification = Image height / Object height
Thus. according to the sign conventions :-
Object is always placed above the principal axis so it will be positive.

1] Real images are always inverted and lie below the principal axis and are thus negative. Therefore, m = - / + = - incase of real images.

2] Virtual images are always erect and lie above the principal axis and are thus positive. Therefore , m = + / + = + incase of virtual images.