(x+1)+(x+2)+(x+3)+...+(x+100)=5150

(x+x+x+...+x)+(1+2+3+...+100)=5150

100.x+(1+2+3+...+100)=5150

Ta xét :

A=1+2+3+...+100

A=100+99+98+...+1

⇒A+A=(1+100)+(2+99)+(3+98)+...+(100+1)

2A=101+101+101+...+101(có 100 số101)

2A=101 . 100

A= 101 . 100 :2

A= 10100:2

A= 5050

⇒ 100.x+(1+2+3+...+100)=5150

100.x + 5050 = 5150

100.x = 5150-5050

100.x=100

x=100:100

x=1

Vậy x = 1

có 1  người vì 3 = bố

a.

a.(b+c)-b.(a-c) = (a+b).c

Ta xét vế trái :

a.(b+c)-b.(a-c)

=a.b +a.c - b.a +b.c

=(a.b+b.c) -(b.a-a.b)

=(a+b).c - 0

=(a+b).c

Vậy a.(b+c)-b.(a-c)= (a+b).c

b.

a.(b-c)-a.(b+d) = -a.(c+d)

Ta xét vế trái :

a.(b-c)-a.(b+d)

=a.b - a.c - a.b - a.d

=(a.b - a.b) - (a.c - a.d)

= 0 - a.(c+d)

= -a.(c+d)

Vậy a.(b-c)-a.(b+d) = -a.(c+d)

❤Good❤ study !!!❤