SOLUTION: find the solution of x^2-21=4x. if the solution is a fraction, enter it in simplest form x= or x=

Algebra ->  Quadratic Equations and Parabolas -> SOLUTION: find the solution of x^2-21=4x. if the solution is a fraction, enter it in simplest form x= or x=      Log On


   

Question 188403: find the solution of x^2-21=4x. if the solution is a fraction, enter it in simplest form
x= or x=
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
x%5E2+-+21+=+4x
Subtract 4x from both sides
x%5E2+-+4x+-+21+=+0
Add 21 to both sides
x%5E2+-+4x+=+21
Use complete the square method
x%5E2+-+4x+%2B+%284%2F2%29%5E2+=+21+%2B+%284%2F2%29%5E2
x%5E2+-+4x+%2B+4+=+21+%2B+4
%28x+-+2%29%5E2+=+25
Take the square root of both sides
x+-+2+=+5
and
x+-+2+=+-5
x+=+7
x+=+-3
check the answers:
x%5E2+-+21+=+4x
7%5E2+-+21+=+4%2A7
49+-+21+=+28
28+=+28
and
x%5E2+-+21+=+4x
%28-3%29%5E2+-+21+=+4%2A%28-3%29
9+-+21+=+-12
-12+=+-12
OK