78. Áp dụng tính chất dãy tỉ số bằng nhau ta có: 
a/b = b/c = c/a = (a + b + c)/(b + c + a) = 1 
Do a/b = 1 => a = b (1) 
Do b/c = 1 => b = c (2) 
Do c/a = 1 => c = a (3) 
Từ (1); (2); (3) => a = b = c.

79. a:b:c:d=2:3:4:5
=>a/2=b/3=c/4=d/5=a+b+c+d/2+3+4+5=-42/14=-3
=>a=-3.2=-6
b=-3.3=-9
c=-3.4=-12
d=-3.5=-15
80. a/2=b/3=c/4, a+2b-3c=-20
ta có a/2=b/3=c/4=2b/6=3c/12=a+2b-3c/2+6-12=-20/-4=5
=>a=10
b=15
c=20
81.
a/2=b/3=> a/10=b/15  ;  b/5=c/4 => b/15=c/12
=>a/10=b/15=c/12=a-b+c/10-15+12=-49/7=-7
=>a=-70
b=-105
c=-84

82. a/2=b/3=c/4=k         a^2-b^2+2c^2=108
a=2k  b=3k    c=4k

a^2-b^2+2c^2=108
=>(2k)^2-(3k)^2+2*(4k)^2=108
=>4k^2-9k^2+32k^2=108
=>27k^2=108
=>k^2=4  =>k=±2
Vậy
a = 4     b = 6     c = 8
a = - 4   b = - 6   c = - 8

a, 2(x-1)^2 =16
b, 9^x+1-5*3^2x = 324