tolong dibantu dongg :'

[tex]\displaystyle a,b,c\text{ barisan geometri sehingga}\\a=a\\b=ar\\c=ar^2\\\\\frac{a-b}{a-2b+c}=\frac{a-ar}{a-2ar+ar^2}\\\frac{a-b}{a-2b+c}=\frac{1-r}{1-2r+r^2}\\\frac{a-b}{a-2b+c}=\frac{1-r}{(1-r)^2}\\\frac{a-b}{a-2b+c}=\frac{1}{1-r}\\\frac{a-b}{a-2b+c}=\frac{1}{1-r}\cdot\frac{ar}{ar}\\\frac{a-b}{a-2b+c}=\frac{ar}{ar-ar^2}\\\boxed{\boxed{\frac{a-b}{a-2b+c}=\frac{b}{b-c}}}[/tex]