ta có:
A=[tex]a.\frac{1}{2}+a.\frac{1}{3}+a.\frac{1}{4}=a.(\frac{1}{2}+\frac{1}{3}+\frac{1}{4})=\frac{-4}{5}.(\frac{6}{12}+\frac{4}{12}+\frac{3}{12})=\frac{-4}{5}.\frac{13}{12}=-\frac{13}{15}[/tex]
Bạn kham thảo nha:
Thay a = [tex]\frac{-4}{5}[/tex] vào A ta có:
[tex]\Rightarrow \frac{-4}{5}.\frac{1}{2}+(\frac{-4}{5}).\frac{1}{3}+(\frac{-4}{5}).\frac{1}{4}[/tex]
[tex]\Leftrightarrow \frac{-4}{5}.\frac{13}{12}[/tex]
[tex]\Rightarrow A= \frac{-13}{15}[/tex]