\(sinEFG=\dfrac{EG}{FG}=\dfrac{3}{8}\)

\(\widehat{EFG\: }\simeq22^o\)

=> Chọn C

+) \(3\left(n+1\right)+11⋮n+3\)

\(11⋮n+3\)

\(n+3\inƯ\left(11\right)=\left\{1;11\right\}\)

\(n=8\)

+) \(3n+16⋮n+4\)

\(3\left(n+4\right)+4⋮n+4\)

\(4⋮n+4\)

\(n+4\inƯ\left(4\right)=\left\{1;2;4\right\}\)

\(n=0\)

+) \(28-7n⋮n+3\)

\(49-7\left(n+3\right)⋮n+3\)

\(49⋮n+3\)

\(n+3\inƯ\left(49\right)=\left\{1;7;49\right\}\)

\(n\in\left\{4;46\right\}\)

\(\dfrac{5-2x}{3}=\dfrac{4x-1}{5}\)

\(5\left(5-2x\right)=3\left(4x-1\right)\)

\(15-10x=12x-3\)

\(25x+10x=15-3\)

\(22=28\)

\(x=\dfrac{28}{22}=\dfrac{14}{11}\)

\(sin\widehat{EFG\: }=\dfrac{EG}{FG}=\dfrac{3}{4}\)

\(\Rightarrow\widehat{EFG\: }\simeq48,6^o\)

\(x^3+3xy+y^3-1\)

\(=x^3+3xy\left(x+y\right)+y^3-3xy\left(x+y\right)+3xy-1\)

\(=\left(x+y\right)^3-1-3xy\left(x+y-1\right)\)

\(=\left(x+y-1\right)\left[\left(x+y\right)^2+\left(x+y\right)+1\right]-3xy\left(x+y-1\right)\)

\(=\left(x+y-1\right)\left(x^2+y^2-xy+x+y+1\right)\)

\(\sqrt{x^2-3x+25}=3\)

\(\left(\sqrt{x^2-3x+25}\right)^2=9\)

\(x^2-3x+25=9\)

\(x^2-3x+16=0\)

\(\Delta=9-4.16=-55\)

Vì \(\Delta< 0\)

=> phương trình vô nghiệm

\(64x^4+y^4\)

\(=64x^4+16x^2y^2+y^4-16x^2y^2\)

\(=\left(8x^2\right)^2+2.8x^2.y^2+y^4-\left(4xy\right)^2\)

\(=\left(8x^2+y^2\right)^2-\left(4xy\right)^2\)

\(=\left(8x^2-4xy-y^2\right)\left(8x^2+4xy-y^2\right)\)

\(3\left(x^4+x^2+1\right)-\left(x^2+x+1\right)^2\)

\(=3\left(x^4+x^2+1\right)-\left(x^4+x^2+1+2x^3+2x^2+2x\right)\)

\(=2\left(x^4+x^2+1\right)-2\left(x^3+x^2+x\right)\)

\(=2\left(x^4+x^2+1-x^3-x^2-x\right)\)

\(=2\left(x^4-x^3-x+1\right)\)

\(=2\left(x^3\left(x-1\right)-\left(x-1\right)\right)\)

\(=2\left(x-1\right)\left(x^3-1\right)\)

\(=2\left(x-1\right)^2\left(x^2+x+1\right)\)

\(a+15\times4=120:2\)

\(a+60=60\)

\(a=60-60\)

\(a=0\)

\(4x^4-32x^2+1\)

\(=4x^4+12x^3+2x^2-12x^3-36x^2-6x+2x^2+6x+1\)

\(=2x^2\left(2x^2+6x+1\right)-6x\left(2x^2+6x+1\right)+\left(2x^2+6x+1\right)\)

\(=\left(2x^2-6x+1\right)\left(2x^2+6x+1\right)\)