Question

tính tổng

a,   A=1.2+2.3+3.4+...+19.20

b,   B=1^2+3^2+5^2+7^2+...+99^2

c,  C=2^2+4^2+6^2+...+20^2

Mk cần gấp 

mấy bạn làm nhanh cho mk nhá

 

Answer 1

A=1.2+2.3+3.4+...+19.20

3A=1.2.3+2.3.3+3.4.3+...+19.20.3

3A=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+19.20.(21-18)

3A=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...19.20.21-18.19.20

3A=1.2.3+2.3.4+3.4.5+...+19.20.21-0.1.2-1.2.3-2.3.4-...-18.19.20

3A=19.20.21-0.1.2

3A=7980-0

3A=7980

A=7980÷3

A=2660

Answer 2

B=1^2+3^2+5^2+7^2+...+99^2

B=1.1+3.3+5.5+7.7+...+99.99

B=1.(2-1)+3.(4-1)+5.(6-1)+7.(8-1)+...+99.(100-1)

B=1.2+3.4+5.6+7.8+...+99.100-(1+3+5+7+...+99)

B=(99.100.101)÷3-(99+1).50

B=333300-5000

B=328300

Answer 3

mk cảm ơn bn nhiều nha trần hương thu