21)
2x/5y + 5y/5y = 13/10
2x/5y + 1 = 13/10
2x/5y = 13/10 - 10/10
2x/5y = 3/10
20x=15y
x/y= 3/4
24) a) 6(x+1)=7x => x=6
b) 9x=7x+14 => x=7
c) 9x=8x+8 => x=8
23) n=1989^2-1989-1988
n= 1989^2-1 echivalent cu n=1989^2-1^2
e formula a^2-b^2=(a+b)(a-b)
Va rezulta ca n= (1989+1)*(1989-1)= 1990*1988
1988*1990/1988/4 = 497/n
1990/4=497/n
n=(4*497)/1990
22) pe 4^993 il vom scrie ca 2^1986
a/x=2^1986/(1/4)
a/x=2^1986+4 echivalent cu a/x=2^1986+2^2
a/x=2^1988
a=2^1988 rezulta ca x=1
