Question

cho A = 1 -2 + 3 - 4 + 5 - 6 + ....... + 19 - 20

a, A co chi het ch2, cho 3, cho 5 khong?

b, tim x biet: 1 + 5 + 9 + 13 + 16 + ..... + x

Answer 1

A = 1 - 2 + 3 - 4 + 5 - 6 + ...... + 19 - 20

   = (1 - 2) + (3 - 4) +( 5 - 6) + ...... + (19 - 20)

   = -1 + (-1) + (-1) + ...... + (-1)

   = -1 x 10

   = -10

Answer 2

A=1-2+3-4+5-6+.....+19-20

A=(1-2)+(3-4)+(5-6)+....+(19-20)

A=(-1)+(-1)+(-1)+...+(-1)  co 10 so -1

=>A= (-1)×10=-10

Ma -10 chia het cho 2 va 5 nhung khong chia het cho 3