Tính bằng cách hợp lý

a)\(\dfrac{7}{15}\)-(\(\dfrac{2}{15}\)-\(\dfrac{12}{18}\))

b) (\(\dfrac{7}{41}\)-\(\dfrac{4}{9}\))-(\(\dfrac{3}{19}\)+\(\dfrac{7}{41}\))+(\(\dfrac{4}{9}\)-\(\dfrac{16}{19}\))

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