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.

Kazu is my best friend.

Washoku is a traditional Japanese cuisine.

This room gives a sense of peace.

This room has a Japanese-style atmosphere.

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

Problems about arithmetic series often appear in math tests.