1-((log2/log5)) = 1/x
(log5/log5)=1
(log5-log2)/log5 =1/x
flip it round to find x (reciprocal)
log5/(log5-log2) = x
It's just neater to write this way than yours.
For trapezium rule question.
b=4 a=1 n=6 (because there are 6 strips and 7 ordinates) there are always one more ordinate than strips, try drawing it out with 6 strips and you'll see what I mean.