Question

se f(2x+3)=x^2 +5x, então f(2): ?

Answer 1

$\mathrm{f(2x+3)=x^2+5x}\\\\ \mathrm{Como\ queremos\ o\ valor\ de\ f(2),\ igualaremos}\\ \mathrm{a\ express\~ao\ (2x+3)\ a\ 2,\ encontrando\ o\ va-}\\ \mathrm{lor\ de\ x\ a\ ser\ utilizado:}\\\\ \mathrm{2x+3=2\ \to\ 2x=2-3}\\\\ \mathrm{2x=-1\ \to\ x=\dfrac{-1}{2}}\\\\ \mathrm{Portanto:}\\\\ \mathrm{f(2)=\bigg(\dfrac{-1}{2}\bigg)^2+5.\bigg(\dfrac{-1}{2}\bigg)=\dfrac{1}{4}-\dfrac{5}{2}=}\\\\\\ \mathrm{=\dfrac{1-10}{4}=\dfrac{-9}{4}\ \to\ \boxed{\mathbf{f(2)=\dfrac{-9}{4}}}}$