Tổng: ` sin^2 2^o + sin^2 4^o + sin^2 6^o + ... + sin^2 84^o + sin^2 86^o + sin^2 88^o + sin^2 90^o ` bằng

* `sinalpha= cos (90^@ - alpha)`

* `sin^2 alpha + cos^2 alpha = 1`

`sin^2 2^@+sin^2 4^@+sin^2 6^@+...+sin^2 84^@+sin^2 86^@+sin^2 88^@+sin^2 90^@`

`= cos^2 88^@ + cos^2 86^@ + ... + cos^2 44^@ + sin^2 44^@ + ... + sin^2 86^@ + sin^2 88^@ + sin^2 90^@`

`= sin^2 90^@ + (cos^2 88^@ + sin^2 88^@) + (cos^2 86^@ + sin^2 86^@) + ... + (cos^2 44^@ + sin^2 44^@)`

`= 1 + 1 + ... + 1`

`= 1 + 22 = 23`