Solve[5 x^2+4 x-27==0,x]

x+3/.(x->y)

a=b

rl=x->y;
x+y/.rl

var={x->1,y->3};
var2={x->2,y->45};
x+y/.var

x+y/.var2

res=Solve[5 x^2+4 x-27==0,x][[1]]

x+y/.res

res=Solve[5. x^2+4 x-27==0,x]

x+y/.res

res=Solve[5. x^2+4 x-27==0,x]

res[[1]]

{{-2.7579652245103192`},{1.9579652245103194`}}

x^2/.res[[1]]

ClearAll[myFcn]

myFcn[x_Integer,y_Integer]:=x+y
myFcn[x_Integer,y_Integer,z_Integer]:=(x+y+z)*100

myFcn[5,4]
myFcn[5,4,1]

x=5
myFcn[x_Integer,y_Integer]:=x+y

myFcn[1,1]

ClearAll[a]

k=6

a[1]=1+k;
a[2]:=1+k

?a

k=3

a[1]

a[2]

fcn[x_]:=x^2

fcn[4]

rg=Range[5]
Sin/@rg

harem={Jane,An,Zuzan}

numbers={4,5,6}

g/@numbers
Map[g,numbers]

sex/@harem

Range[6]

g@@numbers

g[numbers]

sex@@harem
Apply[sex,harem]

Range[100]

Plus@@Range[100]

Plus@@{a,b,c,d}
Apply[Plus,{a,b,c,d}]

Exp

Times@@{a,b,c,d}

ClearAll[u];
u[t_]:={10*(1-(E^((-t)/0.001))),t};
tmax=0.004;
n=100;
step=(tmax/n);
Table[u[t],{t,0,tmax,step}];
ListPlot[%,AxesLabel->{"u","t"}];

(*ListPlot@({10*(1-(E^((-#)/0.001))),#}&/@(4.*(10^(-5))*Range[100]))*)

pl1=Plot[{Cos[x],x},{x,0,(Pi/2)}];
FindRoot[x==Cos[x],{x,0.5}]
Nest[Cos,0.,200]

NestList[g,x,5]

Show

derivative[t_,f_]:=-0.1*Sin[f+t];
initialt=0;
tmax=20.;
n=1000;
Δt=(((tmax-initialt))/n);
initialf=10;
initalState={initialt,initialf};
EulerStep[{t_,f_}]:={t+Δt,f+derivative[t,f]*Δt};
solution=NestList[EulerStep,initalState,n];
ListPlot[solution]

f[x_]:=Exp[x];
x0=2;
tangenta=f[x0]+(x-x0)*f'[x0];
Manipulate[Plot[{f[x],tangenta},{x,x0-Δx,x0+Δx},PlotStyle->Thick],{Δx,0.001,2}]

initalState

step2=EulerStep[initalState]

step3=EulerStep[step2]

step4=EulerStep[EulerStep[EulerStep[initalState]]]

NestList[EulerStep,initalState,3]