Đa thức 

Câu hỏi: Đa thức $P\left(x\right)={\left(1+3x+2x^2\right)}^{10}=a_0+a_{1}x+...+a_{20}.x^{20}$. Tìm $a_{15}$.

A. $a_{15}=C_{10}^{10}.C_{10}^5.3^5+C_{10}^9.C_9^6.3^3+C_{10}^8.C_8^7.3$.

B. $a_{15}=C_{10}^{10}.C_{10}^5.2^5+C_{10}^9.C_9^6.2^6+C_{10}^8.C_8^7.2^7$.

C. $a_{15}=C_{10}^{10}.C_{10}^5.2^5+C_{10}^9.C_9^6.3^3.2^6+C_{10}^8.C_8^7.2^7$.

D.$a_{15}=C_{10}^{10}.C_{10}^5.3^5.2^5+C_{10}^9.C_9^6.2^6+C_{10}^8.C_8^7.3.2^7$.