Question

1:Tim x

( x-3)(x-5)=0

(x+7).35 = 2.35

2 : Tinh tong 

1.2+2.3+3.4+....................+99.100

Answer 1

1) (x-3)(x-5) = 0

\(\Rightarrow\orbr{\begin{cases}x-3=0\\x-5=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=3\\x=5\end{cases}}\)

(x+7).35 = 2.35

\(\Rightarrow x+7=2\)

\(\Rightarrow x=2-7=-5\)

Vậy x = -5

2) 1.2 + 2.3 + 3.4 + .... + 99.100

Đặt A = 1.2 + 2.3 + .... + 99.100

3A = 1.2.3 + 2.3.4 + 3.4.3 + .... + 99.100.3

3A = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2)+ .... + 99.100.(101-98)

3A = ( 1.2.3 + 2.3.4 + 3.4.5 +.... + 99.100.101 ) - ( 0.1.2 + 1.2.3 + 2.3.4 + ... + 98.99.100 )

3A = 99.100.101 - 0.1.2

3A = 999900 - 0

3A = 999900

A = 999900 : 3

A = 333300

Vậy A = 333300

Answer 2

1

\(\left(x-3\right)\left(x-5\right)=0\)

\(\Rightarrow\orbr{\begin{cases}x-3=0\\x-5=0\end{cases}\Rightarrow\orbr{\begin{cases}x=-3\\x=-5\end{cases}}}\)

\(\left(x+7\right).35=2.35\)

\(x+7=2=>x=-5\)

Answer 3

A=1.2+2.3+3.4...+99.100
=> 3A= 1.2.3+2.3.3+3.4.3...+99.100.3
=> 3A=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+99.100(101=98)
=> 3A= (1.2.3+2.3.4+3.4.5+...+99.100.101)-(0.1.2+1.2.3+..+98.99.200)
=> 3A= 99.100.101-0.1.2
=> 3A= 999900-0=999900
=> A= 999900:3=333300