i cannot find all x values from abs(x) using solve() function

2021 9 30
1 个回答
更新时间：2021 10 4
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syms x
y=abs(x);
r=solve(y==1,x);
ans=
r = 1
but x can be 1 as well as -1,why doesnt they show -1 as well

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回答（1 个）

Steven Lord
Steven Lord 2021-9-30
Ran in:

2 个投票

Ran in:
Tell solve to show you the parameterized solution and the conditions that those parameters need to satisfy in order for the parameterized solution to be valid.
syms x
y=abs(x);
r=solve(y==1,x, 'ReturnConditions', true)
r = struct with fields:
x: exp(y*2i) parameters: y conditions: y <= pi & 0 <= y
x isn't the set [1, -1]. But that's because there are complex solutions.
yy = exp(1i/2) % This is r.x with parameter y = 1/4
yy = 0.8776 + 0.4794i
abs(yy)
ans = 1
Or if you only want real solutions tell solve that:
r=solve(y==1,x, 'ReturnConditions', true, 'Real', true)
r = struct with fields:
x: [2×1 sym] parameters: [1×0 sym] conditions: [2×1 sym]
r.x
ans = 

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