A = 1 + -3 + 5 + -7 +..+ 17 + -19

= ( 1 + -3 ) + ( 5 + -7 ) +...+ ( 17 + -19)

= -2 + -2 + ..+ -2

= (-2 ) x 10

= -20

B = ( -2 ) +4 + ( -6 ) +8 + ...+ ( -18 ) + 20

= ( -2 + 4 ) + ( -6 + 8) + ... + ( -18 + 20 )

= 2  +2 + ... +2

= 10 x 2 = 20

C = 1 + -2 + 3 + -4 + ..+ 1999 + -2000 + 2001

= ( 1 + -2 ) + ( 3 + -4 ) + ... + ( 1999 + -2000 ) + 2001

= -1 + -1 + ...+ -1 + 2001

= (-1) x 1000 + 2001

= -2000 + 2001 = 1

K mk nha

1.would you like a drink? - oh yes,...... a cokr. Thank you A. i have B. I'M HAVING c. I'LL HAVE D. i'm going to have

2.i'll keep your application.......file for the time being A.in B.with C.on D.at

3.What do you hope to.....by telling such lies? A.gain B.win C.earn D.mean

4....... the double, she took the victim to the hospital A.in B. for C.on D.with

5. there is construction going on outside today, sp lease .... the window closed A.stay B.remain C. let D.keep

she wasn't strict teacher but chaeting is the one she ....... allow for A. mustn't B. mightn't C. wouldn't D. shouldn't

what is your attitude ....... smoking in public A. toward B. for C. with D. about