We know that the mid point formula is
[tex]x=\frac{x_2+x_1}{2}\text{and y=}\frac{y_2+y_1}{2}[/tex]we will check one by one which option make this formula by substituing values given:-
It is given that
[tex]x=\frac{a}{a+b}(x_2-x_1)+x_1and\text{ y=}\frac{a}{a+b}(y_2-y_1)+y_1[/tex]Now when a=1 anf b=2 we have
[tex]\begin{gathered} x=\frac{1}{3}(x_2-x_1)+x_1 \\ =\frac{x_2-x_1+3x_1}{3} \\ =\frac{x_2+2x_1}{3} \end{gathered}[/tex]which is not the midpoint formula
Now let's substitute a=1 and a+b=2
[tex]\begin{gathered} x=\frac{1}{2}(x_2-x_1)+x_1 \\ =\frac{x_2-x_1+2x_1}{2} \\ =\frac{x_2+x_1}{2} \\ \text{and } \\ y=\frac{1}{2}(y_2-y_1)+y_1 \\ =\frac{y_2-y_1+2y_1}{2} \\ =\frac{y_2+y_1}{2} \end{gathered}[/tex]which is the required formula.
Hence the correct option is a=1 , a+b=2