3 × y × ($\frac{1}{4×7}$ + $\frac{1}{7×10}$ + $\frac{1}{10×13}$ + $\frac{1}{13×16}$) = $\frac{1}{13}$ - $\frac{1}{16}$
y × ($\frac{3}{4×7}$ + $\frac{3}{7×10}$ + $\frac{3}{10×13}$ + $\frac{3}{13×16}$) = $\frac{1}{13}$ - $\frac{1}{16}$
y × ($\frac{1}{4}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{10}$ + $\frac{1}{10}$ - $\frac{1}{13}$ + $\frac{1}{13}$ - $\frac{1}{16}$) = $\frac{1}{13}$ - $\frac{1}{16}$
y × ($\frac{1}{4}$ - $\frac{1}{16}$) = $\frac{1}{13}$ - $\frac{1}{16}$
y × ($\frac{4-1}{16}$) = $\frac{16-13}{208}$
y × $\frac{3}{16}$ = $\frac{3}{208}$
y = $\frac{3}{208}$ : $\frac{3}{16}$
y = $\frac{3}{208}$ × $\frac{16}{3}$
y = $\frac{1}{13}$
Chúc bạn học tốt ^^
