Đáp án:
Giải thích các bước giải:
BT1:
a/ $6n+4n-20=2000$
⇔ $10n=2020$
⇔ $x=202$
b/ $1+3+5+....+n=36$
⇔ $(\frac{n-1}{2}+1)(\frac{n+1}{2})=36$
⇔ $\frac{n+1}{2}.\frac{n+1}{2}=36$
⇔ $\frac{(n+1)^2}{4}=36$
⇔ $(n+1)^2=36.4=144$
⇔ \(\left[ \begin{array}{l}n+1=12\\n+1=-12\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=11\\x=-13\end{array} \right.\)
c/ $2+4+6+8+......+2n=110$
⇔ $(\frac{2n-2}{2}+1)(\frac{2n+2}{2})=110$
⇔ $\frac{2n}{2}.\frac{2n+2}{2}=110$
⇔ $\frac{2n(2n+2)}{4}=110$
⇔ $4n(n+1)=440$
⇔ $n^2+n=110$
⇔ $n^2+n-110=0$
⇔ $n^2-10n+11n-110=0$
⇔ $(n-10)(n+11)=0$
⇔ \(\left[ \begin{array}{l}x=10\\x=-11\end{array} \right.\)
BT2:
a/ $xy-2x=0$
⇔ $x(y-2)=0$
⇔ \(\left[ \begin{array}{l}x=0\\y=2\end{array} \right.\)
b/ $(2x-10).(33-11y)=0$
⇔ $2(x-5).11(3-y)=0$
⇔ $(x-5)(3-y)=0$
⇔ \(\left[ \begin{array}{l}x=5\\y=3\end{array} \right.\)
Chúc bạn học tốt !!
