1) \(\left(+15\right)+\left(+17\right)=15+17=32\)

2) \(\left(-3\right)+\left(-7\right)=-3-7=-\left(3+7\right)=-10\)

3) \(\left(-25\right)+\left(+4\right)=-25+4=-\left(25-4\right)=-21\)

4) \(\left(-6\right)+\left(-54\right)=-6-54=-\left(6+54\right)=-60\)

5) \(\left(-15\right)+20=20-15=5\)

6) \(\left(-5\right)+8+7+5\)

\(=\left(-5+5\right)+\left(8+7\right)\)

\(=15\)

7) \(\left(-8\right)+\left(-11\right)+\left(-2\right)\)

\(=\left[\left(-8\right)+\left(-2\right)\right]+\left(-11\right)\)

\(=\left(-10\right)+\left(-11\right)\)

\(=-21\)

8) \(15+\left(-5\right)+\left(-14\right)+\left(-16\right)\)

\(=\left[15+\left(-5\right)\right]+\left[\left(-14\right)+\left(-16\right)\right]\)

\(=10+\left(-30\right)\)

\(=-20\)

9) \(\left(-20\right)+\left(-14\right)+3+\left(-86\right)\)

\(=\left[\left(-20\right)+3\right]+\left[\left(-14\right)+\left(-86\right)\right]\)

\(=\left(-17\right)+\left(-100\right)\)

\(=-117\)

10) \(\left(-136\right)+123+\left(-264\right)+\left(-83\right)+240\)

\(=\left[\left(-136\right)+\left(-264\right)\right]+\left[123+\left(-83\right)\right]+240\)

\(=\left(-400\right)+40+240\)

\(=\left(-360\right)+240\)

\(=-120\)

11) \(\left(-596\right)+2001+1999+\left(-404+189\right)\)

\(=\left(-596\right)+2001+1999-404+189\)

\(=\left[\left(-596\right)-404\right]+\left(2001+189\right)+1999\)

\(=\left(-1000\right)+2190+1999\)

\(=1190+1999\)

\(=3189\)

12) \(314+\left(-153\right)+64+121+\left(-247\right)+218\)

\(=\left(314+64+121\right)+\left[\left(-153\right)+\left(-247\right)\right]+218\)

\(=\left(378+121\right)+\left(-400\right)+218\)

\(=499-400+218\)

\(=99+218\)

\(=317\)

\(\text{#}Toru\)

a) 5 – 4 – 3 – 2 – 1 + 11 + 12 + 13 + 14 + 15

= (5 + 15) + (14 – 4) + (13 – 3) + (12 – 2) + (11 – 1)

= 20 + 10 + 10 + 10 + 10

= 60 

b) 1 – 2 + 3 – 4 + 5 – 6 + 7 – 8 + 9 – 10 +11

= (1 + 3 + 5 + 7 + 9 + 11) – 2 – 4 – 8 – 10

= 36 – 24

= 12

a) 5 – 4 – 3 – 2 – 1 + 11 + 12 + 13 + 14 + 15

= (5 + 15) + (14 – 4) + (13 – 3) + (12 – 2) + (11 – 1)

= 20 + 10 + 10 + 10 + 10

= 60 

b) 1 – 2 + 3 – 4 + 5 – 6 + 7 – 8 + 9 – 10 +11

= (1 + 3 + 5 + 7 + 9 + 11) – 2 – 4 – 8 – 10

= 36 – 24

= 12

1. 14 + 6 + I -9 I + ( -14 )

= [ ( -14) + 14 ] + 6 + I -9 I

= 0 + 6 + 9

= 15

2.

a) Ix - 3I = 15 

=> x-3= - 15 hoặc x-3=15

*Nếu x-3=-15 

=>    x   = -15 + 3

        x   = -12

*Nếu x-3= 15 

=>    x   = 15+3

        x   = 18

Vậy x=18 hoặc x= -12

b) giải tương tự câu a

c) -3<x<-1

=> x= -2

105

45 k nha

12+ 13+ 14+ 15+ 16 +17 + 18=105

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

12 + 13 + 14 + 15 + 15 + 16 + 17 + 18

= ( 12 + 18 ) + ( 13 + 17 ) + ( 14 + 16 ) + ( 15 + 15 )

= 30 + 30 + 30 + 30

= 30 x 4

= 120

1 + 2 +3 + 4 + 5 + 6 + 7 + 8 + 9

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

= 10 + 10 + 10 + 10 + 5

= 10 x 4 + 5

= 40 + 5

 = 45

bải giải

C.dài: 75 cm

C.rộng 60cm

Diện tích phòng học em là:

75 x 60 =4500 ( cm2 )

đáp số: 4500 cm2 .

vậy bằng bao nhiêu m2 ha bạn

x+1=2

x= 2-1

x=1

1+1=2

=> x=1

\(942\times112-942\times11-942\)

\(=942\times\left(112-11-1\right)\)

\(=942\times100\)

\(=94200\)

\(\dfrac{1}{3}\times\dfrac{14}{15}+\dfrac{14}{15}\times\dfrac{2}{3}\)

\(=\dfrac{14}{15}\times\left(\dfrac{1}{3}+\dfrac{2}{3}\right)\)

\(=\dfrac{14}{15}\times\left(\dfrac{1+2}{3}\right)\)

\(=\dfrac{14}{15}\times\dfrac{3}{3}\)

\(=\dfrac{14}{15}\times1\)

\(=\dfrac{14}{15}\)

    942 \(\times\) 112 - 942 \(\times\) 11 - 942

= 942 \(\times\) 112 - 942 \(\times\) 11 - 942 \(\times\) 1

= 942 \(\times\) ( 112 - 11 - 1)

= 942 \(\times\) 100

= 94200