a. 3x - 5 =4 => 3x = 4+5

                   => 3x = 9  => x = 3

Vậy x = 3

b. 4x + 3x - 1 = 3x => 4x = 1 => x= 1/4

Vây x = 1/4

dấy là làm tắt

công thức 

1. Tính số số hạng có trong tổng:

        SSH = (SLN – SBN) : KC + 1

2. Tính tổng của dãy:

        T = (SLN + SBN) x SSH : 2

2+4+...+100= (100+2)*((100-2):2+1):2

tuoi cua em la 

(25-5):2= 10 

tuoi cua anh la

25-10=15

1) $f\left(x\right)=-3x^2-12x+5$

$\Rightarrow f\left(x\right)=-3\left(x^2+4x\right)+5$

$\Rightarrow f\left(x\right)=-3\left(x^2+4x+4\right)+5+12$

$\Rightarrow f\left(x\right)=-3{\left(x+2\right)}^2+17\le 17\left(-3{\left(x+2\right)}^2\le 0,\forall x\right)$

$\Rightarrow GTLN\left(f\left(x\right)\right)=17\left(tạix=-2\right)$

a2 + b2 + c2 = ab + bc + ca 

<=> 2a2 + 2b2 + 2c2 - 2ab - 2bc - 2ca = 0

<=> (a2 - 2ab + b2) + (b2 - 2ca + c2) + (c2 - 2ac + a2) = 0

<=> (a - b)2 + (b - c)2 + (c - a)2 = 0

Dễ thấy   (a - b)2 + (b - c)2 + (c - a)2 $\ge 0\forall a,b,c$

Dấu "=" xảy ra khi $\{\begin{array}{c}a-b=0\\ b-c=0\\ a-c=0\end{array}\iff a=b=c$

Mà a + b + c = 2025 

nên $a=b=c=675$

$2a^2+8b^2-8ab$

$=2\left(a^2-4ab+4b^2\right)$

$=2{\left(a-2b\right)}^2$

1.I usually have free time on weekend.

2.I usually play sports with my free time.

3. Because for me all these activities have meaning in life

dear my brother 

today i write the email to you about i tell you about i usually do with your friends in your free time. i usually have free time on the weekend. When I have free time, I often watch television. I like watching cartoons very much. Sometimes, I go to the cinema with my friends. we often see the movie together at the weekend. when we see movie, we feel relaxed moreover, I can learn English through the movie.

How about you? What do you do in your free time?

Bye for now,