Question

Tính nhanh :

2/2+2/6+2/12+2/20+2/30+2/42

Ai trả lời trước, mình tick luôn.

Answer 1

Làm

   2/2 + 2/6 + 2/12 + 2/20 + 2/30 + 2/42

= 2/2 + 2/2.3 + 2/3.4 + 2/4.5 + 2/5.6 + 2/6.7 

= 2.( 1/2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 )

= 2. ( 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 )

= 2 . [ ( 1/2 + 1/2 ) + (-1/3 + 1/3 ) + (-1/4 + 1/4 ) + (-1/5 + 1/5 ) + (-1/6 + 1/6 ) - 1/7 ]

= 2 . [ 2/2 - 1/7 ]

= 2 . [ 1 - 1/7 ]

= 2 . [ 7/7 - 1/7 ]

= 2 . 6/7

= 12/7

HỌC TỐT

Answer 2

\(\frac{2}{2}\)+\(\frac{2}{6}\)+\(\frac{2}{12}\)+\(\frac{2}{20}\)+\(\frac{2}{30}\)+\(\frac{2}{42}\)

= \(\frac{2}{1.2}\)+\(\frac{2}{2.3}\)+\(\frac{2}{3.4}\)+\(\frac{2}{4.5}\)+\(\frac{2}{5.6}\)+\(\frac{2}{6.7}\)

= \(\frac{2}{1}\)-\(\frac{2}{2}\)+\(\frac{2}{2}\)-\(\frac{2}{3}\)+\(\frac{2}{3}-\frac{2}{4}\)+\(\frac{2}{4}\)-\(\frac{2}{5}\)+\(\frac{2}{5}\)-\(\frac{2}{6}\)+\(\frac{2}{6}\)-\(\frac{2}{7}\)

=\(\frac{2}{1}\)-\(\frac{2}{7}\)

= \(\frac{12}{7}\)

                            CỐ GẮNG HOK TỐT NHÉ

Answer 3

\(\frac{2}{2}+\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+\frac{2}{30}+\frac{2}{42}\)

\(=\frac{2}{2}+\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+\frac{2}{5.6}+\frac{2}{6.7}\)

\(=\frac{2}{1}-\frac{2}{2}+\frac{2}{2}-\frac{2}{3}+\frac{2}{3}-\frac{2}{4}+\frac{2}{4}-\frac{2}{5}+\frac{2}{5}-\frac{2}{6}\)

\(=\frac{2}{1}-\frac{2}{7}\)

\(=\frac{12}{7}\)