Chọn A

A

A

a) x + 7 = -12

x = (-12) - 7

x = 19

b) x - 15 = -21

x = (-21) + 15

x = -6

c) 13 - x = 20

x = 13 - 20

x = -7

a, x+7=-12

\(\Leftrightarrow\) x= -19

b, x-15=-21

\(\Leftrightarrow\) x= -6

c, 13-x=20

\(\Leftrightarrow\) x=-7

a) x+7=-12\(\Leftrightarrow\) x=-12-7\(\Leftrightarrow\) x=-19

b) x-15=-21\(\Leftrightarrow\) x=-21+15\(\Leftrightarrow\) x=-6

c)13-x=20\(\Leftrightarrow\) -x=20-13\(\Leftrightarrow\)-x=7\(\Leftrightarrow\) x=-7

cái này cấp 1 cũng biết ..........

\(a.\)

\(x+7=-12\)

\(\Rightarrow x=-12-7=-19\)

Vậy \(x=-19\)

\(b.\)

\(x-15=-21\)

\(\Rightarrow x=-21+15=-6\)

Vậy \(x=-6\)

\(c.\)

\(13-x=20\)

\(\Rightarrow x=-13-20=-7\)

Vậy \(x-=-7\)

14A

15B

16B

17C

18B

19B

20C

\(16.\\ \left\{{}\begin{matrix}P+N+E=76\\P=E\\\left(P+E\right)-N=20\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=76\\2P-N=20\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}P=E=Z=24\\N=28\end{matrix}\right.\)

\(20.\\ \left\{{}\begin{matrix}P+E+Z=40\\\left(P+E\right)-N=12\\P=E\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=40\\2P-N=12\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}P=E=Z=13\\N=14\end{matrix}\right.\\ \Rightarrow A=Z+N=13+14=27\left(đ.v.C\right)\)

\(19\\ \left\{{}\begin{matrix}P+E+N=40\\P=E\\\dfrac{N}{P+E}=\dfrac{7}{13}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=40\\\dfrac{N}{2P}=\dfrac{7}{13}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}2P+N=40\\13N-14P=0\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}P=E=Z=13\\N=14\end{matrix}\right.\\ \Rightarrow A=Z+N=13+14=27\left(đ.v.C\right)\\ \Rightarrow KH:^{27}_{13}R\)

=> Chọn B

a. Do (-2;3) là nghiệm của hpt, thay (-2;3) vào hệ ta được:

\(\Rightarrow\left\{{}\begin{matrix}-2a+9=1\\-2+3b=-2\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=4\\b=0\end{matrix}\right.\)

b. Do hệ có nghiệm là (2;-1), thay (2;-1) vào hệ ta được:

\(\Rightarrow\left\{{}\begin{matrix}2a+b=2\\4a-3b=4\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}a=1\\b=0\end{matrix}\right.\)

\(73^2-27^2=\left(73+27\right)\left(73-27\right)\)

                     \(=100\cdot46=4600\)

\(2002^2-2^2=\left(2002-2\right)\cdot\left(2002+2\right)\)

                       \(=2000\cdot2004=4008000\)

hok tốt .