|a+b| ^2= (a+b).(a+b) = |a|^2+2a.b +|b|^2 |a-b|^2=(a-b).(a-b) = |a|^2-2a.b +|b|^2 |a+b| = |a-b| => a.b =0 |a+b|^2 = 25 = |a|^2 +|b|^2 25 = 9 +|b|^2 |b| = 4
