Resposta :

3478elc
A) y= x - 9     crescente  a > 0

   y = 0 ==>  x - 9 = 0 ==> x = 9
   y > 0 ==>  x - 9 . 0 ==> x > 9
   y < 0 ==>  x - 9 < 0 ==> x < 9



B) y= -5 x + 20   decrescente  a< 0

  y = 0 ==>  -5x  + 20 = 0 ==> -5x = - 20(-1)==>5x = 20 ==> x = 4
   y > 0 ==>  -5x  + 20 > 0 ==> -5x > - 20(-1)==>5x < 20 ==> x < 4
   y < 0 ==>  -5x  + 20 < 0 ==> -5x < - 20(-1)==>5x > 20 ==> x > 4


C) y= -2 x + 6   decrescente   a < 0

  y = 0 ==>  -2x  + 6 = 0 ==> -2x = - 6(-1)==>2x = 6 ==> x = 3
   y > 0 ==>  -2x  + 6 > 0 ==> -2x > - 6(-1)==>2x < 6 ==> x < 3
   y < 0 ==>  -2x  + 6 < 0 ==> -2x < - 6(-1)==>2x > 6 ==> x > 3

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