Answer
$1-y^{10}$
Note that standard form would list the variables first in decreasing order of exponent, so another way to write this answer is: $-y^{10}+1$.
Work Step by Step
$$(1-y^5)(1+y^5)$$ $$=(1\times 1)+(1\times y^5)+(-y^5\times 1)+(-y^5\times y^5)$$ $$=1+y^5-y^5-y^{10}$$ $$=1-y^{10}$$
Note that standard form would list the variables first in decreasing order of exponent, so another way to write this answer is: $$-y^{10}+1$$
