435. a) (a + b + c + d)(a + b - c - d); б) (a - b + c + d)(a - b - c - d); в) (a + b - c + d)(a + b + c - d); г) (a - b - c + d)(a - b + c - d).

a) $$(a + b + c + d)(a + b - c - d) = ((a+b)+(c+d))((a+b)-(c+d))=(a+b)^2 - (c+d)^2=a^2+2ab+b^2 - (c^2+2cd+d^2)=a^2+2ab+b^2-c^2-2cd-d^2$$ б) $$(a - b + c + d)(a - b - c - d) = ((a-b)+(c+d))((a-b)-(c+d)) = (a-b)^2 - (c+d)^2=a^2-2ab+b^2-c^2-2cd-d^2$$ в) $$(a + b - c + d)(a + b + c - d) = ((a+b)+(d-c))((a+b)-(d-c)) = (a+b)^2 - (d-c)^2=a^2+2ab+b^2-(d^2-2cd+c^2)=a^2+2ab+b^2-d^2+2cd-c^2$$ г) $$(a - b - c + d)(a - b + c - d) = ((a-b)-(c-d))((a-b)+(c-d))=(a-b)^2-(c-d)^2=a^2-2ab+b^2-(c^2-2cd+d^2)=a^2-2ab+b^2-c^2+2cd-d^2$$ Ответы: a) $$a^2+2ab+b^2-c^2-2cd-d^2$$ б) $$a^2-2ab+b^2-c^2-2cd-d^2$$ в) $$a^2+2ab+b^2-c^2+2cd-d^2$$ г) $$a^2-2ab+b^2-c^2+2cd-d^2$$