a: \(x\in\left\{1;2;3;6;9;18\right\}\)

mà \(x\in B\left(4\right)\)

nên \(x\in\varnothing\)

b: \(x\in\left\{1;2;3;4;6;9;12;18;36\right\}\)

mà x>=12

nên \(x\in\left\{12;18;36\right\}\)

c: \(x\in\left\{0;12;24;36;48;60;72;84;96;108;...\right\}\)

mà 30<=x<=100

nên \(x\in\left\{36;48;60;72;84;96\right\}\)

d: \(x\inƯC\left(28;21\right)\)

\(\Leftrightarrow x\inƯ\left(7\right)\)

hay \(x\in\left\{1;7\right\}\)

bn ko viết dấu ra à?

hễ trả lời thì nhắn tin cho mik nha!

Thế bạn ko biết dịch ra à . Cứ tưởng ai trả lời . Ai ngờ lại đi hỏi mấy câu ngớ ngẩn !

a: \(\Leftrightarrow x-3\inƯ\left(21\right)\)

\(\Leftrightarrow x-3\in\left\{-3;-1;1;3;7;21\right\}\)

hay \(x\in\left\{0;2;4;6;10;24\right\}\)

b: \(\Leftrightarrow x-1\in\left\{1;-1;17\right\}\)

hay \(x\in\left\{2;0;18\right\}\)

c: \(\Leftrightarrow2x-1+4⋮2x-1\)

\(\Leftrightarrow2x-1\in\left\{1;-1\right\}\)

hay \(x\in\left\{1;0\right\}\)

d: \(\Leftrightarrow x^2+x+3⋮x+1\)

\(\Leftrightarrow x+1\inƯ\left(3\right)\)

\(\Leftrightarrow x+1\in\left\{1;3\right\}\)

hay \(x\in\left\{0;2\right\}\)

a) x² - 7x + 16
= (x² - 2x\(\frac{7}{2}\)+ \(\frac{49}{4}\)) + \(\frac{15}{4}\)
= (x - \(\frac{7}{2}\))² + \(\frac{15}{4}\)> 0
b) 3x² - 3x + 1
= [\(\left(\sqrt{3x^2}\right)^2\)- 2.\(\sqrt{3x^2}\).\(\frac{\sqrt{3}}{2}\)+ \(\frac{3}{4}\)] + \(\frac{1}{4}\)
= (\(\sqrt{3x^2}\)- \(\frac{\sqrt{3}}{2}\))² + \(\frac{1}{4}\)> 0
c) -x² + 3x - 5
= -(x² - 3x + 5)
= -(x² - 2x\(\frac{3}{2}\)+ \(\frac{9}{4}\)+\(\frac{11}{4}\))
= -[(x - \(\frac{3}{2}\))² + \(\frac{11}{4}\)] < 0
d) Câu này sai đề rồi bạn ơi