Zhèngmíng zhè ge héngděngshì xūyào yòng dào èrxiàngshì dìnglǐ.
Proving this identity requires the binomial theorem.
Tips
usage
Distinguish 恒等式 (identity, true always: sin²x + cos²x = 1) from 方程 (fāngchéng, equation, true for specific values: x + 1 = 2). Math teachers stress this — students often confuse them.
memory
恒 (constant/always) + 等 (equal) + 式 (formula): the 'always-equal formula'. The middle 等 distinguishes it from 等式 (equality) — adding 恒 makes it eternally equal.
