\(\frac{1}{s}+\frac{1}{s'}=\frac{1}{f}\)

\(\frac{1}{s'_1}=\frac{1}{-40}-\frac{1}{60}=\frac{1}{-24}\)

\(\frac{1}{s'_2}=\frac{1}{-40}-\frac{1}{60+100}=\frac{1}{-32}\)<\p>

\(\Delta L=-24-(-32)=8cm\)