\(a.172x^2-7^9=2^{-3}.98^3=117649\)

\(172x^2=117649+7^9=40471256\)

\(x^2=40471256:172=235298\)

\(x=\sqrt{235298}=485.07......\)

cái gì ? Mk đã học đến \(\sqrt{ }\) cái này đâu 

Devil Girl lớp 7 đâu đã học \(\sqrt{ }\)

a.\(3x^2-51=-24\)

\(\Rightarrow3x^2=27\)

\(\Rightarrow x^2=9\)

\(\Rightarrow x=3\)

b.\(5x.\left(5^3\right)^2=625\)

\(\Rightarrow5x=5^5:5^{\left(3x2\right)}\)

\(\Rightarrow5x=5^5:5^6\)

\(\Rightarrow5x=5^{-1}=0,2\)

\(\Rightarrow x=0,2:5\)

\(\Rightarrow x=0,04\)

Mk làm lộn câu b nha.

\(\left(\frac{-3}{4}\right)^{3x-1}=\frac{256}{81}\)

\(\Rightarrow\left(\frac{-3}{4}\right)^{3x}:\left(\frac{-3}{4}\right)=\frac{256}{81}\)

\(\Rightarrow\left(\frac{-3}{4}\right)^{3x}=\frac{256}{81}.\left(\frac{-3}{4}\right)\)

\(\Rightarrow\left(\frac{-3}{4}\right)^{3x}=\frac{64}{27}\)

\(\Rightarrow\left(-\frac{3}{4}\right)^{3x}=\left(-\frac{4}{3}\right)^3\)

\(\Rightarrow1=\left(-\frac{4}{3}\right)^3.\left(-\frac{4}{3}\right)^{3x}\)

\(\Rightarrow1=\left(-\frac{4}{3}\right)^{-x}\)

\(\Rightarrow1=\left(-\frac{3x}{4}\right)\)

\(\Rightarrow1=-3x:4\)

\(\Rightarrow-3x=4\)

\(\Rightarrow x=-\frac{4}{3}\)

Ta có:

\(\frac{256}{81}=\frac{4^4}{"-3"^4}=\frac{1}{\frac{"-3"^4}{4}}=\frac{1}{"\frac{-3}{4}"^4}="\frac{-3}{4}"^4="\frac{-3}{4}"^{3x-1}\Rightarrow3x-1=-4\Rightarrow3x=-4+1\)

\(=-3\)

\(\Rightarrow x=-3:1=-1\)

tớ làm bừa đấy sai thì thôi nha

sai đề