c. 3(4-2:x)$^{2}$ +15=207 d. 9.4$^{x}$+4 $^{x+2}$ =400

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1. c. 3(4-x) ² +15 = 207

   ⇔3(16 – 8x + x ²) + 15 = 207

   ⇔48 – 24x + 3x ² + 15 = 207

   ⇔63 – 24x + 3x ² – 207 = 0

   ⇔ 3x ² – 24x – 144 = 0

   ⇔ 3(x ² – 8x – 48) = 0

   ⇔ x ² – 8x -48 = 0

   ⇔ (x ² + 4x) – ( 12x + 48)=0

   ⇔ x( x + 4) – 12( x + 4) = 0

   ⇔ ( x + 4)( x-12) = 0

   ⇔ [x + 4 = 0 ⇔ x= -4

   x – 12 = 0 x= 12

   d. 9.4^x + 4^x+2=400

   ⇔ 9.4^x + 4^x.4 ² = 400

   ⇔ 4^x(9 +4 ²) = 400

   ⇔ 4^x.25 = 400

   ⇔ 4^x = 16

   ⇔ 4^x = 4^4

   ⇔ x =4

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