bantuin dong Kk² senior

1. √50 . √20 = (√25.√2).(√4.√5) = (5.√2).(2.√5) = 5√2.2√5 =5.2(√2.√5) = 10√10

2. 2√3 (2√40 + √12) = 2√3 (2(√4.√10) + √4.√3) = 2√3 ((2.2√10) + 2√3)
                                   = 2√3 (4√10 + 2√3) = (2√3.4√10) + (2√3.2√3)
                                   = 2.4(√3.√10) + 2.2(√3.√3) = 8√30 + 4√9 = 8√30 + 4.3
                                   = 8√30 + 12

3. (√2 + √5)(√2 - √5) = (√2.√2) + (√2.(-√5)) + (√5.√2) + (√5.(-√5))
                                    = √4 + (-√10) + (√10) + (-√25) = 2 - √10 +√10 + (-5) = 2 -5
                                    = -3

4. (2√5 - 3√3)(2√5 + 3√3)
     = (2√5.2√5) + (2√5.3√3) + ((-3√3).2√5) + ((-3√3).3√3)
     = 2.2(√5.√5) + 2.3(√5.√3) + (-3).2(√3.√5) + (-3).3(√3.√3)
     = 4√25 + 6√15 + (-6√15) + (-9√9) = 4.5 + 6√15 - 6√15 + (-9.3) = 20 + (-27)
     = 20 - 27 = -7