Salut,
[tex]T_{k+1}=C_n^k\cdot a^{n-k}\cdot b^k=C_{13}^k\cdot\left(\dfrac{\sqrt{a}}3\right)^{13-k}\cdot\left(\dfrac{3}{\sqrt[3]{a}}\right)^{k}=C_{13}^k\cdot3^{2k-13}\cdot a^{\frac{13-k}2-\frac{k}3};\\\\\dfrac{13-k}2-\dfrac{k}3=4\Bigg|\cdot 6\Rightarrow 3(13-k)-2k=24,\ deci\ 39-5k=24,\ deci\ k=3.\\\\Termenul\ este\ deci\ T_4=C_{13}^3\cdot 3^{-7}\cdot a^4,\ coeficientul\ c\breve{a}utat\ este\ \dfrac{C_{13}^3}{3^7}.[/tex]
Green eyes.
