Tính cực nhanh: 4/3x4/7+4/7x4/11+4/11x4/15.

* Hướng dẫn giải

= 4x$\frac{4}{3}$x7+ 4x$\frac{4}{7}$x11+ 4x$\frac{4}{11}$x15

= 4x($\frac{4}{3}$x7 + $\frac{4}{7}$x11+$\frac{4}{11}$x15)

=4x($\frac{1}{3}$-$\frac{1}{7}$+$\frac{1}{7}$-$\frac{1}{11}$+$\frac{1}{11}$-$\frac{1}{15}$)

=4x($\frac{1}{3}$-$\frac{1}{15}$)=4x$\frac{4}{15}$= $\frac{16}{15}$.