質問: if `(1+2x+x^2)^n=sum_(r=0)^(2n)a_r x^r ,t h e na=` `(^n c_2)^2` b. `^n c_rdot^n c_(r+1)` c. `^2n c_r` d. `^2n c_(r+1)` if `(1+2x+x^2)^n=sum_(r=0)^(2n)a_r x^r ,t h e na=` `(^n c_2)^2` b. `^n c_rdot^n c_(r+1)` c. `^2n c_r` d. `^2n c_(r+1)`
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