private void button_square_root_Click(object sender, EventArgs e) { if (chuoi1 != null || chuoi2 != null) { if (!kiemtra) { chuoi1 = TinhToan.canbac(float.Parse(chuoi1)).ToString(); } else { chuoi2 = TinhToan.canbac(float.Parse(chuoi2)).ToString(); } hienthi(); } }