Use a linear approximation (or differentials) to estimate the given number. 1/98

 well you would start at x=100 as 1/100 is apporximately 1/98 

so find the tangent line at x=100 

y= 1/x , dy/dx= -1/ ( 2sqrtx) 

when x=100, dy/dx= m= -1/20 

when x=100, y=1/100 

y-y1=m(x-x1) 

etc, then you would set x=98 and solve for y tell me if this helps