Question 450995
y^4 - 1 is a difference of two squares, 
it is in the form a² - b²,
it factors into the form (a + b)(a - b)


If a² is y^4, then a is y² {the square root of y^4}
If b² is 1, then b is 1 {the square root of 1}


y^4 - 1
= (y² + 1)(y² - 1){factored the difference of two squares into two binomials}
= (y² + 1)(y + 1)(y - 1) {y² - 1 was also a difference of two squares, as before}
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