النتيجة تعادل.

Because ن is a sun letter. In Arabic, when الـ comes before a sun letter, the l sound is not pronounced; it assimilates to the next consonant.

So:

• النتيجة
• written with ال
• pronounced an-natīja

The spelling does not change, only the pronunciation.

Yes, النتيجة is feminine.

A big clue is the ending ة (taa marbūṭa), which very often marks feminine nouns in Arabic. Because it is feminine, words that describe it as an adjective, or verbs referring to it, normally appear in the feminine singular form.

الـ is the Arabic definite article, equivalent to the in English.

So نتيجة means result/outcome, and النتيجة means the result.

On the reading where تعادل is a noun, Arabic is using a nominal sentence. In present tense, Arabic normally leaves out the verb to be.

So Arabic can say:

• النتيجة تعادل

without a separate word for is.

This is very normal in Arabic.
If you wanted past tense, you would usually need something like كانت.

Without vowel marks, تعادل is ambiguous. It can be read in more than one way.

The two most likely possibilities are:

• تَعادُل — a noun meaning draw / tie / equality
• تُعادِلُ — a verb meaning equals / is equivalent to

So the intended meaning and context decide how you should understand it.

If تعادل is the noun, the full form would be:

• النَّتيجةُ تَعادُلٌ

If it is the verb, it would be:

• النَّتيجةُ تُعادِلُ ...

In normal Arabic writing, these short vowels are usually omitted, which is why both possibilities look the same in unvowelled text.

Because ordinary Arabic writing usually does not show short vowels or tanwīn.

So although the fully vowelled form is:

• تَعادُلٌ

it is normally written simply as:

• تعادل

The same is true for النَّتيجةُ, which is usually written just النتيجة.

If تعادل is being used as a noun predicate, it does not have to agree in gender with النتيجة the way an adjective would.

Compare:

• النتيجة تعادل — predicate noun
• النتيجة متعادلة — adjective/participle, so it does show feminine agreement

So the lack of a feminine ending on تعادل is not a problem if it is a noun.

Yes, if تعادل is meant as the noun تَعادُل. In that case, the sentence is complete.

But if تعادل is meant as the verb تُعادِلُ meaning equals, then it usually feels incomplete unless something comes after it, such as a number, amount, or comparison.

So whether it feels complete depends on which reading is intended.

Yes. النتيجة متعادلة is also natural Arabic.

The difference is grammatical:

• النتيجة تعادل uses تعادل as a noun
• النتيجة متعادلة uses متعادلة as an adjective/participle agreeing with the feminine noun النتيجة

Both can be understood similarly, but the structure is different.