a) ta có :
\(31^{111}< 32^{111}=\left(2^5\right)^{111}=2^{555}\)
\(17^{139}>16^{139}=\left(2^4\right)^{139}=2^{556}\)
Vì \(2^{555}< 2^{556}\)
Nên \(31^{111}< 17^{139}\)
vậy \(31^{111}< 17^{139}\)
b) Gọi số cần tìm là : x ( \(x\ne0;x\inℕ\))
Ta có :
x chia 5 dư 3 \(\Rightarrow x+3⋮5\)\(\Rightarrow7x+21⋮35\)
x chia 7 dư 4 \(\Rightarrow x+4⋮7\)\(\Rightarrow5x+20⋮35\)
\(\Rightarrow\left(7x+21\right)-\left(5x+20\right)⋮35\)
\(\Rightarrow7x+21-5x-20⋮35\)
\(\Rightarrow\left(7x-5x\right)+\left(21-20\right)⋮35\)
\(\Rightarrow2x+1⋮35\)
\(\Rightarrow2x+1\in\left\{5;-5;7;-7;35;-35\right\}\)
\(\Rightarrow2x\in\left\{4;-6;6;-8;34;-36\right\}\)
\(\Rightarrow x\in\left\{2;-3;3;-4;17;-18\right\}\)
Vậy \(x=2\)
