SOLUTION: y2-4y-45=o I need help solving this my teacher went through it in class but i still don't get it please go through it step by step. Thank you :)

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: y2-4y-45=o I need help solving this my teacher went through it in class but i still don't get it please go through it step by step. Thank you :)      Log On


   

Question 434435: y2-4y-45=o
I need help solving this my teacher went through it in class but i still don't get it please go through it step by step. Thank you :)
Found 2 solutions by edjones, josmiceli:
Answer by edjones(8007) About Me  (Show Source):
You can put this solution on YOUR website!
y2-4y-45=0
(y-9)(y+5)=0
y=9, y=-5
.
Ed

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
It depends on what method your
teacher was trying to show you
I'll use the method known as
"completing the square"
+y%5E2+-+4y+-+45+=+0+
Add 45 to both sides
+y%5E2+-+4y+=+45+
Take 1%2F2 of the coefficient of y,
square it, and add it to both sides
+y%5E2+-+4y+%2B+%28-4%2F2%29%5E2+=+45+%2B+%28-4%2F2%29%5E2+
+y%5E2+-+4y+%2B+4+=+45+%2B+4+
+y%5E2+-+4y+%2B+4+=+49+
Each side is now a perfect square
+%28+y+-+2+%29%5E2+=+7%5E2+
Take the square root of both sides (both (+) root and (-) root)
+y+-+2+=+7+
(1) +y+=+9+
and
+y+-+2+=+-7+
(2) y+=+-5+
-----------
You can rewrite (1) and (2) as:
(1) +y+-+9+=+0+
(2) +y+%2B+5+=+0+
Now you can factor the equation:
+y%5E2+-+4y+-+45+=+%28y+-+9%29%2A%28y%2B5%29+