How do you solve the quadratic #x^2-(4+i)x+9+7i=0# using any method?
2 Answers
Explanation:
Quadratic formula
The zeros of a quadratic in the form
#x = (-b+-sqrt(b^2-4ac))/(2a)#
In our example:
#x^2-(4+i)x+(9+7i)=0#
we have
#x = ((4+i)+-sqrt((-(4+i))^2-4(1)(9+7i)))/(2*1)#
#=((4+i)+-sqrt((16+8i+i^2)-36-28i))/2#
#=((4+i)+-sqrt(-21-20i))/2#
#=((4+i)+-i sqrt(21+20i))/2#
Sqaure root of a+bi
Note that I found in https://socratic.org/s/aw38evei
that the square roots of
#+-((sqrt((sqrt(a^2+b^2)+a)/2)) + (b/abs(b) sqrt((sqrt(a^2+b^2)-a)/2))i)#
In particular, if
#(sqrt((sqrt(a^2+b^2)+a)/2)) + (sqrt((sqrt(a^2+b^2)-a)/2))i#
So we find:
#sqrt(21+20i)#
#=(sqrt((sqrt(21^2+20^2)+21)/2)) + (sqrt((sqrt(21^2+20^2)-21)/2))i#
#=(sqrt((sqrt(441+400)+21)/2)) + (sqrt((sqrt(441+400)-21)/2))i#
#=(sqrt((sqrt(841)+21)/2)) + (sqrt((sqrt(841)-21)/2))i#
#=(sqrt((29+21)/2)) + (sqrt((29-21)/2))i#
#=(sqrt(50/2)) + (sqrt(8/2))i#
#=(sqrt(25)) + (sqrt(4))i#
#=5+2i#
Conclusion
So:
#x=((4+i)+-i sqrt(21+20i))/2#
#=((4+i)+-i (5+2i))/2#
#=((4+i)+-(-2+5i))/2#
#= { ((2+6i)/2 = 1+3i), ((6-4i)/2 = 3-2i) :}#
Factor
#1+3i# and#3-2i#
Explanation:
Note that if this has roots
#x^2-(4+i)x+(9+7i)#
#= (x-x_1)(x-x_2) = x^2-(x_1+x_2)+x_1 x_2#
So we want to find
#{ (x_1 + x_2 = 4+i), (x_1 x_2 = 9+7i) :}#
Gaussian integers
What are the Gaussian integer factors of
#|| 9 + 7i || = sqrt(9^2+7^2) = sqrt(81+49) = sqrt(130)#
Any Gaussian integer factors of
#130 = 2 * 5 * 13#
Up to a unit factor, the only Gaussian integer with norm
#(9+7i)/(1+i) = ((9+7i)(1-i))/((1+i)(1-i)) = (16-2i)/2 = 8-i#
Gaussian integers with norm
#(8-i)/(2+i) = ((8-i)(2-i))/((2+i)(2-i)) = (15-10i)/5 = 3-2i#
So:
#9+7i = (1+i)(2+i)(3-2i)#
Any unit (
Taking pairs of these factors we find:
#(1+i)(2+i) = 1+3i " "# with co-factor#(3-2i)#
#(2+i)(3-2i) = 8-i " "# with co-factor#(1+i)#
#(1+i)(3-2i) = 6-5i " "# with co-factor#(2+i)#
The first of these sums to
#(1+3i) + (3-2i) = 4+i#
So our zeros are