Chapter 2: Polynomials

Algebraic Identities

The Essential 8 Identities


Identity 1: Square of Sum

(x+y)2=x2+2xy+y2(x + y)^2 = x^2 + 2xy + y^2

Example: (3+2)2=9+12+4=25(3 + 2)^2 = 9 + 12 + 4 = 25


Identity 2: Square of Difference

(xy)2=x22xy+y2(x - y)^2 = x^2 - 2xy + y^2

Example: (52)2=2520+4=9(5 - 2)^2 = 25 - 20 + 4 = 9


Identity 3: Difference of Squares

(x+y)(xy)=x2y2(x + y)(x - y) = x^2 - y^2

Example: (7+3)(73)=499=40(7 + 3)(7 - 3) = 49 - 9 = 40


Identity 4: Product of Two Binomials

(x+a)(x+b)=x2+(a+b)x+ab(x + a)(x + b) = x^2 + (a + b)x + ab

Example: (x+3)(x+5)=x2+8x+15(x + 3)(x + 5) = x^2 + 8x + 15


Identity 5: Square of Trinomial

(x+y+z)2=x2+y2+z2+2xy+2yz+2zx(x + y + z)^2 = x^2 + y^2 + z^2 + 2xy + 2yz + 2zx


Identity 6: Cube of Sum

(x+y)3=x3+y3+3xy(x+y)(x + y)^3 = x^3 + y^3 + 3xy(x + y)

Expanded: =x3+3x2y+3xy2+y3= x^3 + 3x^2y + 3xy^2 + y^3


Identity 7: Cube of Difference

(xy)3=x3y33xy(xy)(x - y)^3 = x^3 - y^3 - 3xy(x - y)

Expanded: =x33x2y+3xy2y3= x^3 - 3x^2y + 3xy^2 - y^3


Identity 8: Sum of Cubes

x3+y3+z33xyz=(x+y+z)(x2+y2+z2xyyzzx)x^3 + y^3 + z^3 - 3xyz = (x + y + z)(x^2 + y^2 + z^2 - xy - yz - zx)

Special Case: If x+y+z=0x + y + z = 0, then x3+y3+z3=3xyzx^3 + y^3 + z^3 = 3xyz!