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my youtube videos under " theme (matelle) "; e.g. formulas change 1 (matelle); Formulas convert 2 (matelle) ; special chain set (matelle) ; interleaved triset (matelle) ; KgV and ggT (matelle) ; trigonometry (matelle) ; Convert units (matelle) ; pq formula (matelle) ; Binomic formulas (matelle) ; addition method (matelle) ; tangent to curve (matelle)