If f(x) = ax + b, where a and b are real numbers. If f( f ( f(x) ) ) = 8x + 21, what is the value of a + b [DCRU]
A. 4
B. 3
C. 5
D. 2

• f(f(f(x)))= (a^3)x+(a^2)b+ab+b = 8x+21
  we have on comparing
  a=2
  b=3
  a+b = 5
• 11 years agoHelpfull: Yes(27) No(0)
• f(x)=ax+b
  f(f(x))=a(ax+b)+b=a^2x+ab+b
  f(f(f(x)))= a^2(ax+b)+ab+b=a^3+a^2b+ab+a=8x+21
  on comparing a^3=8=>a=2
  a^2b+ab+a=21=>b=3
  so a+b=2+3=5
• 11 years agoHelpfull: Yes(10) No(1)
• f(f(f(x)))=a^3x+a^2b+ab+b=8x+21...from that...a=2,b=3..a+b=5
  
• 11 years agoHelpfull: Yes(2) No(0)
• 5 a=2 b=3 so ans is 5
• 11 years agoHelpfull: Yes(0) No(1)
• a+b=5
  a=2, b=3
  
• 11 years agoHelpfull: Yes(0) No(0)
• c..........................
• 11 years agoHelpfull: Yes(0) No(0)
• a^3x+a^2b+ab+b=8x+21. so a=2 & b=3. a+b=5
• 11 years agoHelpfull: Yes(0) No(0)
• f(f(f(x)))= (a^3)x+(a^2)b+ab+b = 8x+21
  we have on comparing
  a=2
  b=3
  a+b = 5
• 11 years agoHelpfull: Yes(0) No(0)
• put 3 times f(x)value and compare with 8x + 21
  and get 5
• 11 years agoHelpfull: Yes(0) No(0)