"First looking at it, it was quite easy, but there are always those questions," said Shreya, who is in year 12 at Cheltenham Girls High School.

Across the state, 32,195 students sat the mathematics general 2 exam on Monday morning, and 17,433 students sat the mathematics exam on Monday afternoon.

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Another 3261 students sat the maths extension 2 exam, the highest-level HSC maths course, on Monday afternoon.

Georgia Kirkpatrick-Jones said the extension 2 exam was "very challenging".

"There were some questions we hadn't seen before. It was a struggle to get through," Georgia, 17, said.

She said she decided to do the university-level course to "get a good head start" on complex maths concepts.

"I thought it was a good challenge especially because I want to do engineering," Georgia said.

"It's very rewarding. I think we've also developed our resilience to get through an entire paper.

"At the beginning of the year, I would have thrown that paper away."

Rosemary Leslie, who has been teaching maths at Cheltenham Girls for 27 years, said she struggled a little herself with the final question of the maths extension 2 test.

"I thought, I'll have to go away and think about this quietly on my own," Mrs Leslie said. "But it was a fair paper."

Mrs Leslie said the maths general 2 and mathematics papers have "become a bit more difficult" over the years.

"With extension 1 and 2, the wording of the questions is different and they give you a few more hints, but difficulty hasn't changed much," Mrs Leslie said.

The school's head of maths, Jonathan Maranik, said this year's exams weren't "any trickier than previous exams".

"There were a couple of questions which I looked at and said, 'there will definitely be some struggles'," Mr Maranik said.

"Like always, there are some questions which are aimed at differentiating them. It's a fairly comprehensive and rigorous test."

Nearly 9000 students will sit the maths extension 1 exam on November 3.

This year, more than 58,500 students - or 75 per cent of the year 12 cohort - are enrolled in at least one maths course.

Maths remains the "most popular HSC elective", but the proportion of year 12 students studying maths is declining.

About 94.2 per cent of HSC students studied a maths subject in 1986. This fell to 82.6 per cent in 2006, and stood at 77.6 per cent in 2016. Enrolments reached a low in 2013 when only 72 per cent of students did at least one maths subject.

There are no plans to make maths a compulsory HSC subject, a NSW Education Standards Authority spokesman said.

English is the only compulsory HSC subject.