Tìm x: a) x . ( x + 1 ) = 20; b) [x . (x + 1)] : 2 = 21; d) 1 + 2 + 3+ 4 + ... + x = 210

Tìm x
a) x . ( x + 1 ) = 20
b) ( x . ( x + 1 ) ) : 2 = 21
d) 1+2+3+4 + ...+ x = 210