Question

1, tính nhanh 

a, (1/2+1/4+1/8+1/16+1/32+1/64+1/128)*1280

b,(2019-2017)+(2015-2013)+...+(7-5)+(3-1)

Answer 1

a ) \(\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\frac{1}{32}+\frac{1}{64}+\frac{1}{128}\right)\times1280\)

= \(\frac{1}{2}\times1280+\frac{1}{4}\times1280+\frac{1}{8}\times1280+\frac{1}{16}\times1280+\frac{1}{32}\times1280+\frac{1}{64}\times1280\)\(+\frac{1}{128}\times1280\)

= 640 + 320 + 160 + 80 + 40 + 20 + 10

= ( 640 + 160 ) + ( 320 + 80 ) + ( 40 + 20 + 10 )

= 800 + 400 + 70

= 1270

Answer 2

b ) ( 2019 - 2017 ) + ( 2015 - 2013 ) + ... + ( 7 - 5 ) + ( 3 - 1 )  (có 505 cặp số )

= 2 + 2 + ... + 2 + 2 ( có 505 số 2 )

= 2 x 505

= 1010