AS surely as two plus two equals four, so mathematics plus science equals modern technology and much modern business. Leave out mathematics and the equation won't work. That's a message America's mathematicians want to get across as they celebrate National Mathematics Awareness Week, April 21-27.

It's a message with the ring of d'ej`a vu. American mathematicians have been alarmed for over a decade at the decline in math skills in the United States.

To focus public attention on the problem, the Joint Policy Board of Mathematics, which represents professional math societies, encouraged President Reagan to proclaim the first math awareness week in 1986.

But there's more than a repeat of old alarms this year. There has been some progress on the math-skill front.

Demand for undergraduate college math courses has increased by more than 70 percent since 1970. New productive math programs have appeared at some institutions.

Citing such progress, a report released this month by a committee of the National Academies of Science and Engineering in Washington concludes that ``national success in mathematics is within reach - given appropriate funding, commitment, and program development,'' as the academy announcement puts it.

Thus math week '91 marks a turning point in meeting the math-skill challenge. The time for hand-wringing within the math community is over. Now the need is for that community to work with the larger educational community to turn math education around.

The academy announcement says it bluntly - ``many American universities and colleges need to make radical changes in the way they teach mathematics to undergraduate students.'' That goes for secondary schools, too.

All too often, math courses seem dull and irrelevant to living for many students. The academy report - prepared by the Committee on the Mathematical Sciences in the Year 2000 - notes that computers, calculators, and other modern teaching tools have yet to make it into most math classrooms.

That's a sorry comment on United States math instruction.

Consider, for example, that the ability to use probability and statistics is becoming an essential business skill. Yet how can students really learn such skill without computer and calculator tools to do the tedious statistical arithmetic that frees them to concentrate on how to use statistical concepts?

The math-skill challenge is as much a matter of student choice as it is of educational resources. The academy report indicates that a change in math educators' attitudes would do much to make math more attractive.

The report rightly points out that student success is impeded by a pervasive and false mythology.

This includes such misperceptions as the belief that innate ability rather than hard work determines math skill, that women and certain ethnic groups are innately less capable mathematically, and that most jobs require little math skill. Making math courses interesting and relevant to students' lives would do much to correct such misperceptions.

The academy report urges math teachers to teach ``as if each student is a national asset.'' OK mathematicians, go for it!