>>45
作図して求めると
https://i.imgur.com/AVOcGdz.png

> (opt=optim(c(2,-2),\(x)f(x[1],x[2]),control=list(fnscale=-1)))
$par
[1] 2.094336 -2.094534

$value
[1] 4.5
最大値4.5

https://i.imgur.com/JQRIuu7.png