FACTORING SPECIAL CASES
The vocabulary of perfect squares Perfect squares are numbers like 4, 9, 16, 25, etc. Any variable to an even power is a perfect square. X 2, Y 4, A 6, D 8 are all perfect squares 4x 2 is a perfect square (2x)(2x) (3y) 2 is a perfect square (3y)(3y) The word ‘difference’ means ‘subtract’
The difference of perfect squares X 2 – 9 (difference of perfect squares) AC number is (1)(9) = 9 B = 0 (no x term) - 9 SUBTRACT You need 2 numbers that multiply to give 9 and subtract to give 0 ( + 3 and – 3 ) X 2 + 3x – 3x – 9 = x(x + 3) – 3(x + 3) (x + 3)( x – 3 )
Shortcut! X 2 – 9 = (x + 3)( x – 3 ) Square root 1st + square root 2 nd Square root 1st - square root 2 nd Pattern is always the same
More examples 4X = (2x + 3)(2x – 3) 25x 2 – 64y 2 = (5x + 8y)(5x - 8y) 36x 2 – 144 = 36(x 2 – 4) = 36(x+2)(x-2) See how much easier it is when you factor out the greatest common factor first!
The sum of perfect squares Unless you can pull out a GCF, they are PRIME 4X Prime 25x y 2 = Prime 36x = 36(x 2 + 4) See how much easier it is when you factor out the greatest common factor first!
Perfect square trinomials Look for the pattern: 4X x + 9 4X 2 is a perfect square (+2x)(+2x) 9 is a perfect square (+3)(+3) (+2x)(+3) = +6x doubled = +12x 4X x + 9 = (2x+3)(2x+3)
Watch your signs! 4X x + 9 4X 2 is a perfect square (+2x)(+2x) 9 is a perfect square (-3)(-3) (+2x)(-3) = -6x doubled = -12x 4X x + 9 = (2x-3)(2x-3)
Look for the pattern! 4X x + 9 First and last are perfect squares Middle is double the product of their roots Last number is always positive Signs in the parentheses match middle sign 4X x + 9 = (2x-3)(2x-3) 4X x + 9 = (2x+3)(2x+3)
Fallback plan These special cases are faster to do if you recognize them and remember the ‘formula’. They can also be solved by the same grouping method that we have used for all other trinomials. 4X = (2x + 3)(2x - 3) 4X PRIME 4X x + 9 = (2x - 3)(2x - 3) 4X x + 9 = (2x + 3)(2x + 3)
Difference of perfect square Sum of perfect squares Perfect square trinomials You can write the answer just by looking at them if you can: recognize them remember the formula They also work out just fine if you do them the old-fashioned way ~ it just takes longer.