Three members GP

The sum of three numbers in GP (geometric progression) is 21 and the sum of their squares is 189. Find the numbers.

Correct result:

a =  3
b =  6
c =  12
a2 =  12
b2 =  6
c2 =  3

Solution:

a+b+c=21 a2+b2+c2=189  b=qa c=q2a  a+qa+q2a=21 a2+q2a2+q4a2=189  a(1+q+q2)=21 a2(1+q2+q4)=189  212(1+q2+q4)=189(1+q+q2)2 252 q4378 q3126 q2378 q+252=0  q4+q2+1=(q2q+1) (q2+q+1)  212(q2q+1) (q2+q+1)=189(1+q+q2)2 212(q2q+1)=189(1+q+q2)  212(q2q+1)=189(1+q+q2) 252q2630q+252=0  a=252;b=630;c=252 D=b24ac=63024252252=142884 D>0  q1,2=b±D2a=630±142884504 q1,2=630±378504 q1,2=1.25±0.75 q1=2 q2=0.5   Factored form of the equation:  252(q2)(q0.5)=0   a=21/(1+q1+q12)=21/(1+2+22)=3

Our quadratic equation calculator calculates it.

b=q1 a=2 3=6
c=q1 b=2 6=12
a2=21/(1+q2+q22)=21/(1+0.5+0.52)=12
b2=q2 a2=0.5 12=6
c2=q2 b2=0.5 6=3



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