θ360\frac{\theta}{360}360θ *π * r2^22 → 210×22×4.2×4.2360×7\frac{210\times 22\times 4.2\times 4.2}{360\times 7}360×7210×22×4.2×4.2
161750\frac{1617}{50}501617 = 32.34cm2^22
