Question

已知等差数列{an}中Sp=Sq(p不等于q),求Sp+q

Answer 1

Sp=p*a1+p*(p-1)*d/2

Sq=q*a1+q*(q-1)*d/2

Sp=Sq

所以,pa1+p*(p-1)*d/2=qa1+q*(q-1)*d/2

化简,得:(p-q)*a1=(q-p)*(p+q-1)*d/2

因为p不等于q

所以a1=-(p+q-1)*d/2

S(p+q)

=(p+q)*a1+(p+q)*(p+q-1)*d/2

=-(p+q)*(p+q-1)*d/2+(p+q)*(p+q-1)*d/2

=0

过程很详细吧~

3L明显乱说..