How do you solve $x^{2} + 3 x - 70 = 0$ $x^2+3x-70=0$ by completing the square?

Question

6020 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2015-07-06T10:47:33

When the $x$ $x$ -coefficient (in this case $3$ $3$ ) is odd, completing the square is not the best method.

Consider the sum-product-method instead: The task is to factor $- 70$ $-70$ into two factors that add up to $3$ $3$ .

After some trying you may find that $10 and - 7$ $10and-7$ will do the job.
Now you can factorise the left part of the equation:

$\to (x + 10) \cdot (x - 7) = 0$ $->(x+10)*(x-7)=0$ so either:
$x + 10 = 0 \to x = - 10 or x - 7 = 0 \to x = 7$ $x+10=0->x=-10orx-7=0->x=7$

How do you solve x2+3x−70=0x^2+3x-70=0 by completing the square?