Life-or at least life on TV-is really just a class in geometry.

There’s “Numb3rs” with Rob Morrow on Fridays, and there used to be “Square Pegs” with Sarah Jessica Parker.

But the most important shape these days is the triangle-the love triangle. From dramas to comedies, many of the most successful shows are employing an age-old dilemma: torn between two lovers.

Why has the triangle suddenly become the shape of television romance?

It’s a way to preserve the dramatic and sexual tension between two characters who are mutually attracted-but who’d suddenly become a lot less intriguing if they ever really got together.

Enter the third party. This keeps things lively-and gives viewers someone to root for or against.

TV’s triangles vary in degree and size. Some are isosceles-two of the three sides are equal in length and made for each other. Some have changing members, some are fixed. Some are TV’s equivalent of forever (longer than a season), while others are resolved. The winning equation is: Tension plus believability equals success. The implausible (and short-lived) Joey-Ross-Rachel triangle on “Friends” didn’t work. The long-running Dawson-Joey-Pacey on “Dawson’s Creek” did.

Here’s a look at some of today’s top prime-time triangles:

‘BONES’

Desired outcome: We all know Bones and Booth are a great match. As Deschanel told us last season, “These are two people who are attracted to each other, but they don’t really think they’re right for each other. ... They belong together. But it will take a long time.”

‘THE OFFICE’

The angles: Jim (John Krasinski)-Pam (Jenna Fischer)-Karen (Rashida Jones).

The history: Last season, the triangle was Jim-Pam-Roy (David Denman), Pam’s lunkish fiance. At season’s end, Jim told Pam he was in love with her, and they kissed. But when the new season opened, we learned that while Pam had broken up with Roy, Jim had transferred to Dunder Mifflin’s Stamford office, where he befriended the fun-loving Karen. When the Stamford office closed, Jim moved back to Scranton-and so did Karen. They’re now dating-and Pam is clearly upset about that.

Desired outcome: Jim + Pam 4EVER. These two are clearly destined to be together. They’re already great friends, which would provide a great foundation for them to become lovers. In the British version of “The Office,” the two corresponding characters did wind up together-but not until the very end of the series. So, better get used to that triangulation.

‘BROTHERS & SISTERS’

The angles: Kitty (Calista Flockhart)-Warren (Josh Hopkins)-Amber (Keri Lynn Pratt).

The history: When Kitty came home to visit her family, she was living with a gorgeous rich guy from New York. Then, she, a conservative commentator, took a job at the political talk show “Red, White and Blue,” where she met raging liberal Warren. Sparks started flying, Kitty had a night of passion with Warren, and soon, Mr. New York was out of the picture. But then, the wide-eyed Amber arrived at the station-and she and Warren were suddenly dating. And now, there’s a new complication. In the Nov. 19 episode, Rob Lowe showed up as conservative Sen. Robert McCallister, whom Kitty interviewed-and will come to consider a romantic prospect.

Desired outcome: Frankly, we’re not heavily invested in this one.

‘DESPERATE HOUSEWIVES’

The angles: Susan (Teri Hatcher)-Mike (James Denton)-Ian (Dougray Scott).

The history: From the moment in the pilot that their eyes met at the repast after Mary Alice’s funeral-and he tried to eat her inedible macaroni-Susan and Mike were a couple to root for. And for about five minutes, they were blissfully happy together. But then came a series of relationship-wrecking misfortunes-he was suspected of murder; she secretly remarried her ex-husband in order to get insurance to pay for desperately needed surgery, which was performed by a doctor who then fell for her; Mike fell into a coma after being mowed down by a crazy dentist (who later married Bree); Susan faithfully visited Mike at the hospital, where she met Ian, whose wife was also in a deep coma. Now, Susan’s with the still-married Ian. When Mike finally came out of the coma, Edie Britt (Nicollette Sheridan)-still sore that Susan had ruined her engagement to Karl-convinced him that they’d been a couple. Now, Mike’s been charged with murder, Edie’s dumped him, and Ian has offered to pay for the very best legal defense for Mike, on one condition-that Susan has nothing more to do with Mike. Ever. Hey, they don’t call them nighttime soaps for nothing.

Desired outcome: Susan and Mike. That’s all, folks.

‘LOST’

The angles: The perennial Jack (Matthew Fox)-Kate (Evangeline Lilly)-Sawyer (Josh Holloway) triangle.

The history: In the pilot, when Kate stitched Jack’s wound, they seemed to be headed for hotsville. But then, there was also an unmistakable chemistry with bad-boy Sawyer. She respected Jack, she was often angry at Sawyer. She kissed one, then she kissed the other. There was briefly a quadrangle, when Sawyer hooked up with Ana Lucia. But then, right after that, Ana Lucia got killed, and we were back to the old familiar triangle. It seemed to have been finally resolved in the fall finale, when Kate slept with Sawyer. But we’re guessing this threesome isn’t really done yet-especially after the look Kate and Jack shared between the pane of glass.

Desired outcome: Hmm. This is a hotly debated, highly subjective issue. Kate and Jack had long been our personal preference, but the Kate-Sawyer combo really grew on us.

‘UGLY BETTY’

The angles: Daniel (Eric Mabius)-Amanda (Becki Newton)-Sofia (Salma Hayek).

The history: Daniel, Betty’s boss, is a promiscuous bachelor whose many conquests include Amanda, the pretty, catty receptionist at Mode magazine. While she tries to pretend she’s just as casual about their sex as Daniel is, Amanda is secretly in love with him. Deep down, she’s got a heart-and it’s been broken since he’s fallen for Sofia, an editor from a sister publication who’s only visiting for a while-and happens to be engaged to someone else.

Desired outcome: Daniel and Sofia. That’s a grown-up relationship that will help Daniel mature. (But we’re not sure how available Hayek will be to keep appearing in the telenovela she produces.)

‘GREY’S ANATOMY’

The angles: This show has had sooo many-starting with the Meredith (Ellen Pompeo)-Derek (Patrick Dempsey)-Addison (Kate Walsh) trio.

The history: Where do we even begin? Probably in the pilot, when Meredith slept with Derek and soon discovered he was Dr. Derek Shepherd (aka McDreamy), her new surgeon-supervisor at Seattle Grace Hospital. Triangle alert! At the end of the first full season, Mrs. Addison Shepherd (Kate Walsh), also a brilliant doctor, showed up. Turned out there was a triangular back story: Derek had left New York after discovering Addison was cheating on him with his best friend, Mark (aka McSteamy, a plastic surgeon played by Eric Dane). The following season, Derek and Addison worked on their marriage, even though Meredith had asked Derek to “pick me.” The heartbroken intern eventually moved on to handsome veterinarian Finn (Chris O’Donnell). But at the hospital prom that ended last season-if you’re not a “Grey’s” fan, don’t even ask-Derek and Meredith slept together in an examining room, creating a Derek-Meredith-Finn triangle. This time, it was Meredith’s turn to make a choice-a cliffhanger season finale. This season, she eventually picked Derek. But McSteamy-now a regular-has since created other triangles and threatens to keep doing so. There are more triangles than that, but we’ve run out of room.

Desired outcome: McSteamy has many fans-as did Finn-but we’re hoping Meredith and Derek will go the distance.