r[i] = q[i+1]*r[i+1] + r[i+2]　　より
r[i+2] = r[i] - q[i+1]*r[i+1]

s'*r[i+1] + t'*r[i+2] = r[n] 　　に r[i+2]=〜を代入すると
s'*r[i+1] + t'*(r[i] - q[i+1]*r[i+1]) = r[n]
t'*r[i] + (s'-q[i+1])*r[i+1] = r[n]　　を得る.

s*r[i] + t*r[i+1] = r[n]　　の係数と比べてみましょう