次の式を因数分解せよ。 [47~50] (2) (xy+1)(x+1)(y+1)+xy (3) (x-1)(x-3)(x-5)(x-7)+15 (4) (x+1)(x+2)(x+3)(x+6)-3x² 47 (1) (x2+2x) (x²+2x-4)+3 48 (1) 81x-16y4 49 (1) x+x²+1 (2) x 4-5x2 y2-36y4 (2) x-7x2 y2+y4