a)
`1/(2.3)+1/(3.4)+...+1/(49.50)`
`⇔1/2-1/3+1/3-1/4+...+1/49-1/50`
`⇔1/2-1/50`
`⇔12/25`
d)
`1/20+1/30+1/42+...+1/90`
`⇔1/(4.5)+1/(5.6)+...+1/(9.10)`
`⇔1/4-1/5+1/5-1/6+...+1/9-1/10`
`⇔1/4-1/10`
`⇔3/20`
g)
`5/(2.1)+4/(1.11)+3/(11.2)+1/(2.15)+13/(15.4)`
`⇔7 ( 5/ (2.7)+4/(7.11) + 3/(11.14)+1/(14.15)+13/(15.28) )`
`⇔7 (1/2-1/7+1/7-1/11+1/11-1/14+1/14-1/15+1/15-1/28)`
`⇔7 (1/2-1/28)`
`⇔7 . 13/28 `
`⇔13/4 `