1+3=(1+3)*2/2;1+3+5=(1+5)*3;1+3+5+7=(1+7)*4/2````,则1+3+5+7+9+```+(2n-1)等于?
an*n
b(n+1)*(n+1)
c(1+n)*n/2
d(n+1)*n/2
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