Finding envelope of family of curves with two parameters

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enter image description hereConsider this $x/a+y/b=1$, where the parameters $a,b$ are related by $a^2+b^2=c$, where $c$ is a constant. Here they consider $a,b$ as a function of some other parameter $t$ and proceed to differentiate with respect to $t$? Why is it so? Why should they be a function of $t$ enter image description here