$($Past Exam Question from August $2017.)$ The defining equations of the output of a neural network

are as follows.

$y_i=b_i+w_{i1}{x}_1+w_{i2}{x}_2,i=1,2,$dots$9$

$y{y}_j=b{b}_j+{\displaystyle \underset{i=1}{\overset{9}{}}}w{w}_{ji}{y}_i,j=1,2,$dots$7$

$yyy=bbb+{\displaystyle \underset{i=1}{\overset{7}{}}}ww{w}_{i}y{y}_i$

Write down the back propagation update for $w_{81}.($No need to show your derivation. Try to do it using

your intuition. If you prefer use a diagram. I am only interested in your final answer.$)$