The whole chapter not understand
Kindly refer the similar link for your query.
https://www.meritnation.com/cbse/class6/studymaterial/math/math/basic-geometrical-ideas/1_4_8128
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https://www.meritnation.com/cbse/class6/studymaterial/math/math/basic-geometrical-ideas/1_4_8128
Regards