If α and β are the roots of x² − 6x + 4 = 0, then √α + √β equals

α + β = 6 and αβ = 4; both roots are positive (sum and product positive, discriminant 20 > 0), so the square roots are real.

(√α + √β)² = α + β + 2√(αβ) = 6 + 2 × 2 = 10.

Hence √α + √β = √10.

Trap: '10' forgets to take the square root; √8 comes from using α + β − 2√(αβ) (that gives (√α − √β)² = 2, not 8).