This problem is very difficult.

It's difficult to calculate the sum of this series.

He was able to solve the complex calculation quickly.

He was calculating in his head.

This room gives a sense of peace.

Washoku is a traditional Japanese cuisine.

Kazu is my best friend.

This room has a Japanese-style atmosphere.

Calculating infinite series is one of the more difficult parts of mathematics.

It's difficult to calculate the sum of this convergent series.