Hi Dingyu Xue
Both expressions are the same
t=[-10:.1:10];
y1=2./((t + 2).^2 + 1).^2 - (2*(2*t + 4).^2)./((t + 2).^2 + 1).^3;
y2=-2*(3*t.^2+12*t+11)./(t.^2+4*t+5).^3;
.
Despite the logical evaluation returns null
isequal(y1,y2)
ans =
logical
0
but it's because of really small decimals discrepancy
when plotting
both curves fall right on same place, y1 is same as y2.
MATLAB help for simplify suggests
Simplification of mathematical expression is not a clearly defined subject. There is no universal idea as to which form of an expression is simplest. The form of a mathematical expression that is simplest for one problem might be complicated or even unsuitable for another problem.
Kind of, the function has probably undergone some kind of improvement, the resulting simplification is now split into different fractions, perhaps helping spot poles and zeros, yet the comment clearly remarks that symbolic simplification is not like finding zeros.
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thanks in advance
John BG