Does Stephen Hawking's voice synthesizer handle Math equations?

He does it all in his mind.

His voice box synthesizer is so old it can bearly handle the voice thing let alone anything more complicated.

He has a calculator program.

The communication device that he uses is essentially a computer, capable of running Windows. So it can run any software that’s Windows compatible. He inputs data using a customizable on-screen keyboard. The cursor scans through the keyboard, and he selects the desired inputs by clicking a hand-held switch.

So anything you can type, he can type. Converting that string of characters to speech would be problematic, but there’s likely no need for that, since the output could easily be printed, emailed or projected if need be.