The original formula = lim [x→1] (x-1)/sin (x-1) * (x-m) =1* 3.

∵x→ 1，

∴ 1-m=3

∴m=-2，

According to Vieta's theorem,

1+m=-a，

1*m=b，

∴a= 1，

b=-2。

2. When x→2, the arctangent function is kπ+π/2.

So the arc tangent at x=2 is undefined, it is a discontinuous point, and it is the second kind of removable discontinuous point.

3、y'=[(lnx+ 1)( 1+x^2)-2x*xlmx]/( 1+x^2)^2

=( 1+lnx+x^2-x^2lnx)/( 1+x^2).