a) x²+ 4 = 0

x² = -4

x = √-4

x = √4i²

x = 2i e x = -2i

S = { -2i, 2i }

b) 2x²– 6x + 9 = 0

Δ = b² – 4ac

Δ = (-6)² - 4.2.9

Δ = 36 - 72

Δ = -36

x' = ( -(-6) + √-36 )/2.2 ==> ( 6 + √36i² )/4 ==> ( 6 + 6i )/4 ==> 2.( 3 + 3i )/4 ==> ( 3 + 3i )/2

x'' = ( -(-6) - √-36 )/2.2 ==> ( 6 - √36i² )/4 ==> ( 6 - 6i )/4 ==> 2.( 3 - 3i )/4 ==> ( 3 - 3i )/2

S = { ( 3 - 3i )/2, ( 3 + 3i )/2 }

c) 3x²– 4x + 25 = 0

Δ = b² – 4ac

Δ = (-4)² - 4.3.25

Δ = 16 - 300

Δ = -284

x' = ( -(-4) + √-284 )/2.3 ==> ( 4 + √284i² )/6 ==> ( 4 + √2².71.i² )/6 ==> ( 4 + 2i√71 )/6 ==> 2.( 2 + i√71 )/6 ==> ( 2 + i√71)/3

x'' = ( -(-4) - √-284 )/2.3 ==> ( 4 - √284i² )/6 ==> ( 4 - √2².71.i² )/6 ==> ( 4 + 2i√71 )/6 ==> 2.( 2 + i√71 )/6 ==> ( 2 - i√71)/3

S = { ( 2 - i√71)/3, ( 2 + i√71)/3 }