Выражения: 42ab-(3a+7b)²=? (x+7y)²-14xy=? 25t²-(2z+5t)²=? (t-4)²-(4-t)(t+4)=? (3t-5)²-25=?

42ab - (9a^2 + 2*3*7*ab + 49b^2) = 42ab - 9a^2 - 42ab - 49b^2 = - (9a^2 + 49b^2)
x^2 + 14xy + 49y^2 - 14xy =  x^2 + 49y^2
25t^2 - (4z^2 + 20tz + 25t^2) = 25t^2 - 4z^2 - 20tz - 25t^2 = - 4z^2 - 20tz = - 4z (z + 5t)  
t^2 - 8t +16 - (16 + t^2) = t^2 - 8t +16 - 16 - t^2 = -8t 
9t^2 - 30t + 25 - 25 = 9t^2 - 30t = t (9t - 30)