Question

\(1.\)Tính nhanh

(1 +2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ) x 2 =

50 x 2 + 2 x 10 =

Answer 1

[(1+9)+(2+8)+(3+7)+(4+6)+5]x2

=(10+10+10+10+5)x2

=45x2

=90

50x2+2x10

=2x(50+10)

=2x60

=120

Answer 2

\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)

Answer 3

( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ) x 2 = 45 x 2 = 90 

50 x 2 + 2 x 10 = 2 x ( 50 + 10 ) = 2 x 60 = 120 
 

Answer 4

(1 + 2+3+4+5+6+7+8+9)x2

=45x2

=90

50x2+2x10

=(50+10)x2

=60x2

=120

Answer 5

( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ) . 2 

= 50 . 2 + 2 . 10

= 2 . ( 50 + 10 )

= 2 . 60

= 120

Answer 6

(1+2+3+4+5+6+7+8+9)x 2=70

50x2+2x10=120

Answer 7

=(10+10+10+10+5)x2

=45x2 

=90

Answer 8

a, (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9) x 2

= (1 + 9) + (2 + 8) + (3 + 7) + (4 + 6)  (+ 5) x 2

= 10        +     10    +    10    +    10     +  5   x 2

= 45 x 2

= 90

b, 50 x 2 + 2 x 10

= 2 x (50 + 10)

= 2 x 60

= 120

Answer 9

(1+2+3+4+5+6+7+8+9)x2

=2+4+6+8+10+12+14+16+18

=(18+2)+(4+16)+(6+14)+(8+12)+10

=20+20+20+20+10

=20x4+10

=80+10

=90

50x2+2x10

=2x(50+10)

=2x60

=120

Answer 10

90,120

Answer 11

(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ) x 2 

= [(1+9)+(8+2)+(7+3)+(6+4)+5] x 2 

= (10 + 10 + 10 + 10 + 5) x 2

= 45 x 2 = 90

50 x 2 + 2 x 10 = 100 + 20 = 120

Answer 12

(1+2+3+4+5+6+7+8+9 )=45

45 x 2 = 90

50 x 2 + 2 x 10 = 100