ไม่แน่ใจว่าจะตอบตรงคำถามรึเปล่านะครับ
$(r_1+r_2+\dots+r_{90})^2=(r_1+r_2+\dots+r_{90})(r_1+r_2+\dots+r_{90})$
$=r_1(r_1+r_2+\dots+r_{90})+r_2(r_1+r_2+\dots+r_{90})+\dots+r_{90}(r_1+r_2+\dots+r_{90})$
$=(r_1^2+r_2^2+\dots+r_{90}^2)+2\displaystyle{\sum_{i<j}{r_ir_j}}$
$=(r_1^2+r_2^2+\dots+r_{90}^2)+2S$
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