(1/8z^2 - 9/10)^2
= ((1/8z^2 - 9/10) * (1/8z^2 - 9/10))
= (1/8z^2 1/8z^2) - (1/8z^2 9/10) - (9/10 1/8z^2) + (9/10 9/10)
= 1/64z^4 - 9/80z^2 - 9/80z^2 + 81/100
= 1/64z^4 - 9/40z^2 + 81/100
(1/8z^2 - 9/10)^2
= ((1/8z^2 - 9/10) * (1/8z^2 - 9/10))
= (1/8z^2 1/8z^2) - (1/8z^2 9/10) - (9/10 1/8z^2) + (9/10 9/10)
= 1/64z^4 - 9/80z^2 - 9/80z^2 + 81/100
= 1/64z^4 - 9/40z^2 + 81/100