26.定義:若一個(gè)多項(xiàng)式的各項(xiàng)系數(shù)之和為7的整數(shù)倍,則稱這個(gè)多項(xiàng)式為“7倍系數(shù)多項(xiàng)式”,稱這個(gè)多項(xiàng)式的各項(xiàng)系數(shù)之和為“7倍系數(shù)和”.
例如:多項(xiàng)式20x+8y的系數(shù)和為20+8=28=7x4,所以多項(xiàng)式20x+8y是“7倍系數(shù)多項(xiàng)式”,它的“7倍系數(shù)和”為28.
請(qǐng)根據(jù)這個(gè)定義解答下列問題:
(1)在下列多項(xiàng)式中,屬于“7倍系數(shù)多項(xiàng)式”的是
:(在橫線上填寫序號(hào))
①2x
2-9x;
②3a+5b;
③19x
2-4x+2y-3xy.
(2)若多項(xiàng)式4mx-ny是關(guān)于x.y的“7倍系數(shù)多項(xiàng)式”(其中m,n均為整數(shù)),則多項(xiàng)式2mx+3ny也是關(guān)于x,y的“7倍系數(shù)多項(xiàng)式”嗎?若是,請(qǐng)說(shuō)明理由;若不是,請(qǐng)舉出反例.