R8 is 2.23354 megohms, 1.5% above 2M2
R9 is 1.07018 megohms, 7% above 1M
R10 is 511.60 kilohms, 2.3% above 500k
Knowing a trick amounts to saying "the algebra is not so hard once you've done the hard bit (or someone else has done it for you)"
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Cheers,
GJ