ADMICRO
Tính tổng sau: S=12C0n−14C1n+16C3n−18C4n+…+(−1)n2(n+1)CnnS=12C0n−14C1n+16C3n−18C4n+…+(−1)n2(n+1)Cnn
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ZUNIA12
Lời giải:
Báo saiTa có: S=12(C0n−12C1n+13C2n−…+(−1)nn+1Cnn)S=12(C0n−12C1n+13C2n−…+(−1)nn+1Cnn)
Vì (−1)kk+1Ckn=(−1)kn+1Ck+1n+1(−1)kk+1Ckn=(−1)kn+1Ck+1n+1 nên:
S=12(n+1)∑nk=0(−1)kCk+1n+1=−12(n+1)(∑n+1k=0(−1)kCkn+1−C0n+1)=12(n+1)S=12(n+1)∑nk=0(−1)kCk+1n+1=−12(n+1)(∑n+1k=0(−1)kCkn+1−C0n+1)=12(n+1)
ZUNIA9
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