`1)`
`a) 28,5xx1,5=42,75`
`b) 15,3+20+64,7=(15,3+64,7)+20=80+20=100`
`c) 4,25xx57,43-325+42,57xx4,25`
`= 4,25xx(57,43+42,57)-325`
`= 4,25xx100-325`
`= 425-325`
`=100`
`d) 12/50+8%+59/100+9%`
`= 24%+8%+59%+9%=100%`
`2)`
`a)x+9,44=18,36`
`x=18,36-9,44`
`x=8,92`
`b)2012:x+23=526`
`2012:x=526-23`
`2012:x=503`
`x=2012:503`
`x=4`
`c)(x-1/2)xx5/2+1/2=7/4`
`(x-1/2)xx5/2=7/4-1/2`
`(x-1/2)xx5/2=5/4`
`x-1/2=5/4:5/2`
`x-1/2=1/2`
`x=1/2+1/2`
`x=1`
`3)`
`A= 1/2+5/6+11/12+19/20+29/30+41/42+55/56+71/72+89/90`
`=1-1/2+1-1/6+1-1/(12)+1-1/(20)+1-1/(30)+1-1/(42)+1-1/(56)+1-1/(72)+1-1/(90)`
`=9 – (1/2+1/6+1/(12)+1/(20)+1/(30)+1/(42)+1/(56)+1/(72)+1/(90))`
`=9 – (1/(1.2)+1/(2.3)+1/(3.4)+1/(4.5)+1/(5.6)+1/(6.7)+1/(7.8)+1/(8.9)+1/(9.10))`
`=9 – ( 1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/(10))`
`=9 – (1 – 1/(10))`
`= 9 – 9/(10)`
`= (81)/(10)`
