y = 4sin √ x ( đk x ≥ 0 ) ta thấy -1 ≤ sin √ x ≤ 1 <=> -4 ≤ 4sin √ x ≤ 4 <=> -4 ≤ y ≤ 4 max y = 4 dấu "=" xảy ra <=> sin √ x = 1 <=> √ x = pi/2 +2kpi <=> x = (pi/2 +2kpi )^2 min y = -4 dấu "=" xảy ra <=> sin √ x = -1 <=> √ x = -pi/2 +2kpi <=> x = (-pi/2 +2kpi)^2
