设公差为x.
a1+(a1+2x)+(a1+7x)=(a1+3x)²
3 a1+9x=a1²+2x 3 a1+9x²
(6x-3) a1=9x(9x-1)+a1²
(6x-3-a1) a1=9x(9x-1)
a1=-1 6x-2=81x²-9x
15x-2=81x²
接下来由楼主自己解答.
设公差为x.
a1+(a1+2x)+(a1+7x)=(a1+3x)²
3 a1+9x=a1²+2x 3 a1+9x²
(6x-3) a1=9x(9x-1)+a1²
(6x-3-a1) a1=9x(9x-1)
a1=-1 6x-2=81x²-9x
15x-2=81x²
接下来由楼主自己解答.