Proof of the order of known method

Slavik_edi

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Joined
Jul 25, 2022
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1
Hello everyone,
I'm trying to prove the order of this method:

1658738482504.png


I trying to develop this method by Taylor expansion, but I get a wrong answer.
The method has to be 2nd order, but I get that this is 1st order.
Please help,
Thanks.
 
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