tính nhanh a,A=5-10+15-20+…+2005-2010
b,1-2-3+4+5-6-7+8+…+2000+2001-2002
tính nhanh a,A=5-10+15-20+…+2005-2010 b,1-2-3+4+5-6-7+8+…+2000+2001-2002
By aihong
By aihong
tính nhanh a,A=5-10+15-20+…+2005-2010
b,1-2-3+4+5-6-7+8+…+2000+2001-2002
Bạn tham khảo :
$A= 5 – 10 + 15 – 20 +…+2005-2010$
$A= (5 – 10 )+ (15 – 20) +…+(2005-2010)$
$A= -5 . 201$
$A = -1005$
$1 – 2 – 3 + 4 +5 – 6 – 7 + 8 +…+ 2000 + 2001 – 2002$
$=(1 – 2 – 3)+( 4 +5 – 6 – 7) +(8 +9 – 10 – 11) + …+( 1996 + 1997 – 1998 – 1999)+( 2000+ 2001 – 2002)$
$= 500 . (-4)+ 1999$
$= -2000 + 1999$
$= -1$
a/ A= 5 – 10 + 15 – 20 +…+2005-2010
A= (5 – 10 )+ (15 – 20) +…+(2005-2010)
201 cặp
A= -5 . 201
A = -1005
Vậy A = – 1005
b/1 – 2 – 3 + 4 +5 – 6 – 7 + 8 +…+ 2000 + 2001 – 2002
=(1 – 2 – 3)+( 4 +5 – 6 – 7) +(8 +9 – 10 – 11) + …+( 1996 + 1997 – 1998 – 1999)+( 2000+ 2001 – 2002)
500 cặp
= -4 . 500 + 1999
= -2000 + 1999
= -1