Question

how first equation come
please tell i am practicising​

Answer 1

Answer:

AC² = AF² + CF²

⇒AC² = (AB - BF)² + CF²

⇒AC² = (AB - BF)² + (BC²  - BF²)

⇒AC² = AB² - 2AB.BF + BF² + BC² - BF²

⇒AC² = AB² + BC² - 2AB.BF

I hope it helps

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