(a - 19.99) : 2 = 1 x 1 + 1 x 2 + 1 x 3 + 1 x 4 +...+ 9 x 7 + 9 x 8 + 9 x 9

(a - 19.99) : 2 = 1 x (1 + 2 + 3 + 4 + 5 + 6 + 7 )

Á Xl, mình ấn nhầm nhé '-' Câu trl k đúng đâu !!

A=1x2+2x3+3x4+...+49x50

3A= 3(1.2+2.3+3.4+...+49.50)

3A= 1.2.3+2.3.3+3.4.3+...+49.50.3

3A= 1.2.(3-0)+2.3(4-1)+3.4(5-2)+...+49.50.(51-48)

3A= 0.1.2-1.2.3+1.2.3-2.3.4+2.3.4-3.4.5+...+48.49.50-49.50.51

3A= 49.50.51

A= 49.50.51/3=41650

B=1x3+3x5+5x7+...+99x101

B=1/1.3 +1/3.5 +...+1/99.101

2B=2/1.3 + 2/3.5 +...+2/99.101

2B=1-1/3+1/3-1/5+...+1/99-1/101

2B=1-1/101

2B=100/101

B=100/101:2=100/202

1 + 1 x 2 +1 x 0 +1 x 1

=1 + 2 + 0 + 1

= 3 + 0 + 1

= 3 + 1

= 4

=1

k di

=1+2+3+5

=11 nhé !

1*1+ 1* 2 + 3* 1 + 5 = 1 + 2 + 3 + 5

= 11

\(1+1\div1\times1+1-1\div1\times2=1\)

1 + 1 : 1 x 1 + 1 - 1 : 1 x 2

= 1

1x1+2=3

Vậy 1x2+1=3

Bằng 3 nha 

k mk và nhớ kb ^^

3       

1+1x2=3

vậy : 1x1+2=3

Chọn A

lim x → − 1 x 3 + 1 x 2 + x = lim x → − 1 x + 1 x 2 − x + 1 x x + 1 = lim x → − 1 x 2 − x + 1 x = − 3