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(z+40)(z-20)=90
We move all terms to the left:
(z+40)(z-20)-(90)=0
We multiply parentheses ..
(+z^2-20z+40z-800)-90=0
We get rid of parentheses
z^2-20z+40z-800-90=0
We add all the numbers together, and all the variables
z^2+20z-890=0
a = 1; b = 20; c = -890;
Δ = b2-4ac
Δ = 202-4·1·(-890)
Δ = 3960
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}$$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}$
The end solution:
$\sqrt{\Delta}=\sqrt{3960}=\sqrt{36*110}=\sqrt{36}*\sqrt{110}=6\sqrt{110}$$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(20)-6\sqrt{110}}{2*1}=\frac{-20-6\sqrt{110}}{2} $$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(20)+6\sqrt{110}}{2*1}=\frac{-20+6\sqrt{110}}{2} $
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