Question

a, 1/3.4+1/4.5+..........+1/20.21

b, 1/4.6 + 1/6.8 +.............+ 1/30.32

Answer 1

a,ta thay hai so o duoi mau so la hai sop tu nhien lien tiep khoang cach chinh bang 1 

ta co 1/3-1/4+1/4-1/5+.............................+1/20-1/21

ta co =1/3-1/21 vi co cac so doi to da the hien tren 

=2/7

b vi khoang cach duoi mau kac tu mau la 2 con tu la 1 vay nhan 2 vao ca day so ta duoc 

2/4.6+2/6.8+..............................+2/30.32

bay gio khoang cach duoi mau bang tu ta co

1/4-1/6+1/6-1/8+............................+1/30-1/32

nhu tren ta co =(1/4-1/32):2=7/64