3.
1) 3а⁴-5а³+2а-7а⁴-3а³+8а²-7 = -4а⁴-8а³+8а²+2а-7;
2) 5х³-2х³у+3у²+2х³+ху²+8у²-1 = 7х³-2х³у+ху²+11у²-1;
3) 4а⁵-8а⁴b-4b³+8a⁴b-2a⁵-b³-2 = 2а⁵-5b³-2;
4) 4x⁴+6xy²-5y²-2xy²-3x⁴+5y²-7x = х⁴+4ху²-7х.