`~rai~`
\(A=\sin^210^\circ+\sin^220^\circ+...+\sin^280^\circ\\\quad=\sin^210^\circ+\sin^220^\circ+\cdots+\sin^240^\circ+\cos^240^\circ+\cos^230^\circ+\cdots+\\\quad\cos^210^\circ\\\quad=(\sin^210^\circ+\cos^210^\circ)+(\sin^220^\circ+\cos^220^\circ)+\cdots+\\\quad(\sin^240^\circ+\cos^240^\circ)\\\quad=1+1+1+1\\\quad=4.\\\text{Giải thích:Công thức áp dụng:}\\\sin\alpha=\cos(90^\circ-\alpha)\\\sin^2\alpha+\cos^2\alpha=1.\)
