câu c

C nha em

\(1)3x^2-2x=0\\ \Leftrightarrow x\left(3x-2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\3x-2=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=\dfrac{2}{3}\end{matrix}\right.\)

\(2)2x^2-10=0\\ \Leftrightarrow2x^2=10\\ \Leftrightarrow x^2=5\\ \Leftrightarrow\left[{}\begin{matrix}x=-\sqrt{5}\\x=\sqrt{5}\end{matrix}\right.\)

\(3)x^2-49=0\\ \Leftrightarrow x^2=49\\ \Leftrightarrow\left[{}\begin{matrix}x=7\\x=-7\end{matrix}\right.\)

\(4)x^2-7x+6=0\\ \Leftrightarrow x^2-x-6x+6=0\\ \Leftrightarrow x\left(x-1\right)-6\left(x-1\right)=0\\ \Leftrightarrow\left(x-1\right)\left(x-6\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x-1=0\\x-6=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=1\\x=6\end{matrix}\right.\)

359

\(2\left(x+1\right)^2+3y^2=21\)

Ta có: x,y nguyên

=>\(\left(x+1\right)^2;y^2\) là các số chính phương

mà \(2\left(x+1\right)^2+3y^2=21\) 

nên \(\left[2\left(x+1\right)^2;3y^2\right]\in\left\{\left(18;3\right)\right\}\)

=>\(\left(\left(x+1\right)^2;y^2\right)\in\left(9;1\right)\)

=>\(\left(x+1;y\right)\in\left\{\left(3;-1\right);\left(3;1\right);\left(-3;-1\right);\left(-3;1\right)\right\}\)

=>\(\left(x;y\right)\in\left\{\left(2;-1\right);\left(2;1\right);\left(-4;-1\right);\left(-4;1\right)\right\}\) 

a, \(f\left(2\right)=\left(a+1\right)2-3a+5.4-2=-a+20=12\Leftrightarrow a=8\)

b, \(f\left(-1\right)=\left(a+1\right)\left(-1\right)-3a+3=-9\Leftrightarrow-4a+2=9\Leftrightarrow a=-\dfrac{7}{4}\)

Đặt \(A=1+\dfrac{1}{3}+\dfrac{1}{9}+...+\dfrac{1}{729}+\dfrac{1}{2187}\)

=>\(A=1+\dfrac{1}{3}+\dfrac{1}{3^2}+...+\dfrac{1}{3^6}+\dfrac{1}{3^7}\)

=>\(3A=3+1+\dfrac{1}{3}+...+\dfrac{1}{3^5}+\dfrac{1}{3^6}\)

=>\(3A-A=3+1+\dfrac{1}{3}+...+\dfrac{1}{3^5}+\dfrac{1}{3^6}-1-\dfrac{1}{3}-...-\dfrac{1}{3^7}\)

=>\(2A=3-\dfrac{1}{3^7}=\dfrac{3^8-1}{3^7}\)

=>\(A=\dfrac{3^8-1}{3^7\cdot2}\)

21. The boy ate (A) too much (B) candy and he had (C) a terrible teethache (D)

22. Could (A) you please give (B) me some informations (C) about (D) the common cold?

23.Hoa buyed some meat and a lot of fruit yesterday.

24. My brother doean’t like durians, and I don’t like them, too.

25. Viet not got up early and do morning exercises yesterday.

26. Last week the dentist felt a cavity in my broken tooth.

27.You look tired. You’d better staying inside at recess.

28. Nam is having a medical check-up and so I am.

chọn rồi thây :))

A

A