A=1-2+3-4+5-6+...+199-200(200so hang)

A=(1-2)+(3-4)+(5-6)+.....+(199-200)(100nhom)

A=-1+(-1)+(-1)+.....+(-1) (100 SO (-1))

A=-100

VAY A=-100 LA GTRI CAN TIM

S=2^2+2^3+2^4+2^5+.....+2^20

2S= 2^3+2^4+2^5+.....+2^20+2^21

SUY RA S=2^21-2^2

S=2097148

co 2 cso la0;5